Enhance error log for Network is unreachable due to prefer-ipv6

[michaelortmann]

Found by:
Patch by: michaelortmann
Fixes:

One-line summary:
Enhance error log for Network is unreachable due to prefer-ipv6

Additional description (if needed):
Eggdrop has no automatic fallback regarding prefer-ipv6 nor does it implement https://en.wikipedia.org/wiki/Happy_Eyeballs
Both is not trivial to implement in eggdrop, and discussion in #433 shows its also not trivial to design, so this PR helps out with enhanced logging that is helpful but cheap.

Test cases demonstrating functionality (if applicable):

.console +d
.set prefer-ipv6 1
.jump irc.freenode.net
[00:32:10] Trying server irc.freenode.net:6667
[00:32:10] net: open_telnet_raw(): idx 5 host irc.freenode.net ip 2001:19f0:6401:1697:5400:3ff:fe75:91f2 port 6667 ssl 0
[00:32:10] Failed connect to irc.freenode.net (Network is unreachable)
[00:32:10] may be related to prefer-ipv6 set to 1

.set prefer-ipv6 0
.jump irc.freenode.net
[00:35:15] Trying server irc.freenode.net:6667
[00:35:15] net: open_telnet_raw(): idx 4 host irc.freenode.net ip 149.28.246.185 port 6667 ssl 0
[...]
[00:35:15] net: connect! sock 9
[00:35:15] Connected to irc.freenode.net