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

Author: OlCe2

sys/rpc/auth.h

@@ -354,6 +354,10 @@ __END_DECLS
 #define RPCSEC_GSS	6		/* RPCSEC_GSS */
 #define	AUTH_TLS	7		/* Initiate RPC-over-TLS */
 
+/* RFC 5531's prescribed limits for variable-lenth arrays. */
+#define AUTH_SYS_MAX_HOSTNAME	255
+#define AUTH_SYS_MAX_GROUPS	16	/* Supplementary groups. */
+
 /*
  * Pseudo auth flavors for RPCSEC_GSS.
  */