formatting: add .clang-format config file

Fvwm has had a long history, with contributions from many people over
the years.  As a result the code formatting which has crept in is varied
-- both in terms of whitespace, indentation, etc., with there being mass
inconsistencies all over the code base.

Even as far back as 2006 (see commit:
1c9e17f), there was an attempt at
reformatting some of FvwmButton's code, but not all of it.

Thankfully, technology exists to bring consistency to this
automatically.  clang-format (from llvm) is one example, but it seems to
do the job well enough.

This definition cribs from FreeBSD's style, which isn't too far off the
codebase as it stands at the moment.

.clang-format

@@ -1,33 +1,183 @@
----
+# Basic .clang-format
+---
+BasedOnStyle: WebKit
 AlignAfterOpenBracket: DontAlign
+AlignArrayOfStructures: Left
 AlignConsecutiveAssignments: true
 AlignConsecutiveDeclarations: true
-AlignEscapedNewlinesLeft: true
-AlignOperands: true
+AlignEscapedNewlines: Left
+AlignOperands: false
 AlignTrailingComments: true
-AllowShortBlocksOnASingleLine: false
+AllowAllArgumentsOnNextLine: false
+AllowAllParametersOfDeclarationOnNextLine: false
+AllowShortBlocksOnASingleLine: Never
 AllowShortCaseLabelsOnASingleLine: false
-AllowShortFunctionsOnASingleLine: None
-AllowShortIfStatementsOnASingleLine: false
+AllowShortFunctionsOnASingleLine: InlineOnly
+AllowShortIfStatementsOnASingleLine: Never
 AllowShortLoopsOnASingleLine: false
-AlwaysBreakAfterReturnType: All
-BasedOnStyle: LLVM
+AlwaysBreakAfterReturnType: TopLevelDefinitions
+AlwaysBreakBeforeMultilineStrings: false
+AlwaysBreakTemplateDeclarations: MultiLine
 BinPackArguments: true
 BinPackParameters: true
-BreakAfterJavaFieldAnnotations: true
-BreakBeforeBinaryOperators: NonAssignment
-BreakBeforeBraces: Mozilla
-ColumnLimit: 80
-ContinuationIndentWidth: 4
+BreakBeforeBinaryOperators: None
+BreakBeforeBraces: WebKit
+BreakBeforeTernaryOperators: false
+# TODO: BreakStringLiterals can cause very strange formatting so turn it off?
+BreakStringLiterals: false
+# Prefer:
+# some_var = function(arg1,
+#    arg2)
+# over:
+# some_var =
+#     function(arg1, arg2)
+PenaltyBreakAssignment: 100
+# Prefer:
+# some_long_function(arg1, arg2
+#     arg3)
+# over:
+# some_long_function(
+#     arg1, arg2, arg3)
+PenaltyBreakBeforeFirstCallParameter: 100
+CompactNamespaces: true
 DerivePointerAlignment: false
-ExperimentalAutoDetectBinPacking: false
-IndentWidth: 8
+DisableFormat: false
+ForEachMacros:
+  - ARB_ARRFOREACH
+  - ARB_ARRFOREACH_REVWCOND
+  - ARB_ARRFOREACH_REVERSE
+  - ARB_FOREACH
+  - ARB_FOREACH_FROM
+  - ARB_FOREACH_SAFE
+  - ARB_FOREACH_REVERSE
+  - ARB_FOREACH_REVERSE_FROM
+  - ARB_FOREACH_REVERSE_SAFE
+  - BIT_FOREACH_ISCLR
+  - BIT_FOREACH_ISSET
+  - CPU_FOREACH
+  - CPU_FOREACH_ISCLR
+  - CPU_FOREACH_ISSET
+  - FOREACH_THREAD_IN_PROC
+  - FOREACH_PROC_IN_SYSTEM
+  - FOREACH_PRISON_CHILD
+  - FOREACH_PRISON_DESCENDANT
+  - FOREACH_PRISON_DESCENDANT_LOCKED
+  - FOREACH_PRISON_DESCENDANT_LOCKED_LEVEL
+  - MNT_VNODE_FOREACH_ALL
+  - MNT_VNODE_FOREACH_ACTIVE
+  - RB_FOREACH
+  - RB_FOREACH_FROM
+  - RB_FOREACH_SAFE
+  - RB_FOREACH_REVERSE
+  - RB_FOREACH_REVERSE_FROM
+  - RB_FOREACH_REVERSE_SAFE
+  - SLIST_FOREACH
+  - SLIST_FOREACH_FROM
+  - SLIST_FOREACH_FROM_SAFE
+  - SLIST_FOREACH_SAFE
+  - SLIST_FOREACH_PREVPTR
+  - SPLAY_FOREACH
+  - LIST_FOREACH
+  - LIST_FOREACH_FROM
+  - LIST_FOREACH_FROM_SAFE
+  - LIST_FOREACH_SAFE
+  - STAILQ_FOREACH
+  - STAILQ_FOREACH_FROM
+  - STAILQ_FOREACH_FROM_SAFE
+  - STAILQ_FOREACH_SAFE
+  - TAILQ_FOREACH
+  - TAILQ_FOREACH_FROM
+  - TAILQ_FOREACH_FROM_SAFE
+  - TAILQ_FOREACH_REVERSE
+  - TAILQ_FOREACH_REVERSE_FROM
+  - TAILQ_FOREACH_REVERSE_FROM_SAFE
+  - TAILQ_FOREACH_REVERSE_SAFE
+  - TAILQ_FOREACH_SAFE
+  - VM_MAP_ENTRY_FOREACH
+  - VM_PAGE_DUMP_FOREACH
+IndentCaseLabels: false
+IndentPPDirectives: None
 Language: Cpp
-NamespaceIndentation: All
+NamespaceIndentation: None
 PointerAlignment: Right
-PointerBindsToType: false
-ReflowComments: true
-SortIncludes: true
+ContinuationIndentWidth: 4
+IndentWidth: 8
 TabWidth: 8
+ColumnLimit: 80
 UseTab: Always
-...
+SpaceAfterCStyleCast: false
+IncludeBlocks: Regroup
+IncludeCategories:
+  - Regex: '^\"config\.h\"'
+    Priority: 1
+    SortPriority: 1
+  - Regex: '^\"opt_.*\.h\"'
+    Priority: 1
+    SortPriority: 10
+  - Regex: '^<sys/cdefs\.h>'
+    Priority: 2
+    SortPriority: 20
+  - Regex: '^<sys/types\.h>'
+    Priority: 2
+    SortPriority: 21
+  - Regex: '^<sys/param\.h>'
+    Priority: 2
+    SortPriority: 22
+  - Regex: '^<sys/systm\.h>'
+    Priority: 2
+    SortPriority: 23
+  - Regex: '^<sys.*/'
+    Priority: 2
+    SortPriority: 24
+  - Regex: '\"libs\/.*\.h\"'
+    Priority: 3
+    SortPriority: 15
+  - Regex: '^<[^/].*\.h'
+    Priority: 9
+    SortPriority: 90
+  - Regex: '^\".*\.h\"'
+    Priority: 10
+    SortPriority: 100
+# LLVM's header include ordering style is almost the exact opposite of ours.
+# Unfortunately, they have hard-coded their preferences into clang-format.
+# Clobbering this regular expression to avoid matching prevents non-system
+# headers from being forcibly moved to the top of the include list.
+# http://llvm.org/docs/CodingStandards.html#include-style
+IncludeIsMainRegex: 'BLAH_DONT_MATCH_ANYTHING'
+# FIXME: We want SortIncludes to be "true", however, enabling this exposes
+# bugs in how headers in fvwm are including each other, leading to compilation
+# failures.  Disable for now until this is fixed.
+SortIncludes: false
+KeepEmptyLinesAtTheStartOfBlocks: true
+TypenameMacros:
+  - ARB_ELMTYPE
+  - ARB_HEAD
+  - ARB8_HEAD
+  - ARB16_HEAD
+  - ARB32_HEAD
+  - ARB_ENTRY
+  - ARB8_ENTRY
+  - ARB16_ENTRY
+  - ARB32_ENTRY
+  - LIST_CLASS_ENTRY
+  - LIST_CLASS_HEAD
+  - LIST_ENTRY
+  - LIST_HEAD
+  - QUEUE_TYPEOF
+  - RB_ENTRY
+  - RB_HEAD
+  - SLIST_CLASS_HEAD
+  - SLIST_CLASS_ENTRY
+  - SLIST_HEAD
+  - SLIST_ENTRY
+  - SMR_POINTER
+  - SPLAY_ENTRY
+  - SPLAY_HEAD
+  - STAILQ_CLASS_ENTRY
+  - STAILQ_CLASS_HEAD
+  - STAILQ_ENTRY
+  - STAILQ_HEAD
+  - TAILQ_CLASS_ENTRY
+  - TAILQ_CLASS_HEAD
+  - TAILQ_ENTRY
+  - TAILQ_HEAD