Add FEATURE() definitions for IPv4 and IPv6 so that we can use

feature_present(3) to dynamically decide whether to use one or the
other family.

Reviewed by:	gnn
Sponsored by:	The FreeBSD Foundation
Sponsored by:	iXsystems
MFC after:	10 days

sys/netinet/in_proto.c

@@ -106,6 +106,8 @@ static struct pr_usrreqs nousrreqs;
 #include <net/if_pfsync.h>
 #endif
 
+FEATURE(inet, "Internet Protocol version 4");
+
 extern	struct domain inetdomain;
 
 /* Spacer for loadable protocols. */

sys/netinet6/in6_proto.c

@@ -133,6 +133,7 @@ __FBSDID("$FreeBSD$");
 /*
  * TCP/IP protocol family: IP6, ICMP6, UDP, TCP.
  */
+FEATURE(inet6, "Internet Protocol version 6");
 
 extern	struct domain inet6domain;
 static	struct pr_usrreqs nousrreqs;