kern: fail getgroup and setgroup with negative int

[imp]

Summary

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, without which this change would be a nop.

PR: 189941
Tested by: imp@

[imp]

Not in this review: fixing t_getgroups.c and the regenerated system calls headers.

[markj]

sys/kern/kern_prot.c

300

I would squash these into one line.

805

Same here.