Port commit-message checking to FreeBSD 9.

This fixes a bug reported by Jan Djärv in:
http://lists.gnu.org/archive/html/emacs-devel/2014-12/msg00704.html
along with some other issues I noticed while testing with FreeBSD.
* build-aux/git-hooks/commit-msg: Prefer gawk if available.
Prefer en_US.UTF-8 to en_US.utf8, as it's more portable.
Work around bug in FreeBSD 9 awk, where /[[:cntrl:]]/ matches
ordinary text characters.
Be less tricky about quoting "'" in a shell script.