get mfile/mslice working under Windows

See RELEASE_NOTES.md for details. Two breaking changes are

• findPathPattern convenience function moved from cligen/mfile to cligen/osUt (invisible if you import both...)
• Very low-level (but still exported/exposed) mfile.mopen(fd: cint; st: Stat, ...) changed signature to mopen(fd, fh: cint; fi: FileInfo, ...).