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.

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

(cherry picked from commit f7c4f800cc0b4fac1c99cda8e22d46b67592f9fa)
(cherry picked from commit 37aeac44647dbf2c194c70a56a3fd52aef365aba)