getopt_long(3): note an inconsistency with getopt(3) in BUGS

getopt_long(3) will not allow an optind setting of 0 to be bug-for-bug
compatible with the GNU implementation, as some software does rely on
it. Document it as a BUG, since it affects previous declarations of
compatibility with getopt(3).

Reviewed by: pauamma (markup)
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D37867