sys/rpc: UNIX auth: Use AUTH_SYS_MAX_{GROUPS,HOSTNAME} as limits (2/2)
Remove local defines from 'svc_auth_unix.c' and use the new limit
macros instead.
Reviewed by: rmacklem
MFC after: 2 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52963