sys/rpc: UNIX auth: Use AUTH_SYS_MAX_{GROUPS,HOSTNAME} as limits (1/2)

Consistently with the XDR_INLINE() variant of xdr_authunix_parms()
(_svcauth_unix() in 'svc_auth_unix.c'), reject messages with credentials
having a machine name length in excess of AUTH_SYS_MAX_HOSTNAME or more
than AUTH_SYS_MAX_GROUPS supplementary groups, which do not conform to
RFC 5531. This is done mainly because we cannot store excess groups
anyway, even if at odds with the robustness principle ("be liberal in
what you accept").

While here, make sure the current code is immune to AUTH_SYS_MAX_GROUPS
changing value (in future RFCs?) even if that seems improbable.

Reviewed by: rmacklem
Fixes: dfdcada31e79 ("Add the new kernel-mode NFS Lock Manager.")
MFC after: 2 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52962