kern: fail getgroup and setgroup with negative int

kern: fail getgroup and setgroup with negative int

Found using
https://github.com/NetBSD/src/blob/trunk/tests/lib/libc/sys/t_getgroups.c

getgroups/setgroups want an int and therefore casting it to u_int
resulted in `getgroups(-1, ...)` not returning -1 / errno = EINVAL.

imp@ updated syscall.master and made changes markj@ suggested

PR:			189941
Tested by:		imp@
Reviewed by:		markj@
Pull Request:		https://github.com/freebsd/freebsd-src/pull/407
Differential Revision:	https://reviews.freebsd.org/D30617