Add 32-bit compat for ioctls that take struct ifgroupreq.

Use an accessor to access ifgr_group and ifgr_groups.

Use an macro CASE_IOC_IFGROUPREQ(cmd) in place of case statements such
as "case SIOCAIFGROUP:". This avoids poluting the switch statements
with large numbers of #ifdefs.

Reviewed by:	kib
Obtained from:	CheriBSD
MFC after:	1 week
Sponsored by:	DARPA, AFRL
Differential Revision:	https://reviews.freebsd.org/D14960

newlib/libc/sys/rtems/include/net/if.h

@@ -517,8 +517,10 @@ struct ifgroupreq {
 		char	ifgru_group[IFNAMSIZ];
 		struct	ifg_req *ifgru_groups;
 	} ifgr_ifgru;
+#ifndef _KERNEL
 #define ifgr_group	ifgr_ifgru.ifgru_group
 #define ifgr_groups	ifgr_ifgru.ifgru_groups
+#endif
 };
 
 /*