sys/rpc: Define AUTH_SYS_MAX_{GROUPS,HOSTNAME}

As, respectively, the maximum number of "supplementary" groups and the
maximum hostname size allowed in the credentials structure for AUTH_SYS
(aka, AUTH_UNIX).

Will be used in subsequent commits.

Reviewed by: rmacklem
MFC after: 2 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52961