getgroups: work around FreeBSD bug FreeBSD 7.2 mistakenly succeeds on getgroups(-1,ptr) (POSIX requires EINVAL failure since -1 is less than the proper result). * lib/getgroups.c (rpl_getgroups): Work around the bug. * m4/getgroups.m4 (gl_FUNC_GETGROUPS): Detect the bug. * doc/posix-functions/getgroups.texi (getgroups): Document it. * tests/test-getgroups.c (main): Fix buffer overrun. Signed-off-by: Eric Blake <ebb9@byu.net>