Conditionally compile in the af_inet and af_inet6, af_nd6 modules.

If compiled in for dual-stack use, test with feature_present(3)
to see if we should register the IPv4/IPv6 address family related
options.

In case there is no "inet" support we would love to go with the
usage() and make the address family mandatory (as it is for anything
but inet in theory).  Unfortunately people are used to
  ifconfig IF up/down
etc. as well, so use a fallback of "link".  Adjust the man page
to reflect these minor details.

Improve error handling printing a warning in addition to the usage
telling that we do not know the given address family in two places.

Reviewed by:	hrs, rwatson
Sponsored by:	The FreeBSD Foundation
Sponsored by:	iXsystems
MFC after:	2 weeks

sbin/ifconfig/Makefile

@@ -15,10 +15,16 @@ SRCS=	ifconfig.c		# base support
 #     of the toolchain.
 #
 SRCS+=	af_link.c		# LLC support
+.if ${MK_INET_SUPPORT} != "no"
 SRCS+=	af_inet.c		# IPv4 support
+.endif
+.if ${MK_INET6_SUPPORT} != "no"
 SRCS+=	af_inet6.c		# IPv6 support
+.endif
 SRCS+=	af_atalk.c		# AppleTalk support
+.if ${MK_INET6_SUPPORT} != "no"
 SRCS+=	af_nd6.c		# ND6 support
+.endif
 
 SRCS+=	ifclone.c		# clone device support
 SRCS+=	ifmac.c			# MAC support
@@ -38,6 +44,12 @@ SRCS+=	ifpfsync.c		# pfsync(4) support
 SRCS+=	ifbridge.c		# bridge support
 SRCS+=	iflagg.c		# lagg support
 
+.if ${MK_INET6_SUPPORT} != "no"
+CFLAGS+= -DINET6
+.endif
+.if ${MK_INET_SUPPORT} != "no"
+CFLAGS+= -DINET
+.endif
 .if ${MK_IPX_SUPPORT} != "no" && !defined(RELEASE_CRUNCH)
 SRCS+=	af_ipx.c		# IPX support
 DPADD+=	${LIBIPX}

sbin/ifconfig/af_inet.c

@@ -200,5 +200,7 @@ static struct afswtch af_inet = {
 static __constructor void
 inet_ctor(void)
 {
+	if (!feature_present("inet"))
+		return;
 	af_register(&af_inet);
 }

sbin/ifconfig/af_inet6.c

@@ -541,6 +541,9 @@ inet6_ctor(void)
 #define	N(a)	(sizeof(a) / sizeof(a[0]))
 	size_t i;
 
+	if (!feature_present("inet6"))
+		return;
+
 	for (i = 0; i < N(inet6_cmds);  i++)
 		cmd_register(&inet6_cmds[i]);
 	af_register(&af_inet6);

sbin/ifconfig/af_nd6.c

@@ -225,5 +225,9 @@ static struct afswtch af_nd6 = {
 static __constructor void
 nd6_ctor(void)
 {
+
+	if (!feature_present("inet6"))
+		return;
+
 	af_register(&af_nd6);
 }

sbin/ifconfig/ifconfig.8

@@ -28,7 +28,7 @@
 .\"     From: @(#)ifconfig.8	8.3 (Berkeley) 1/5/94
 .\" $FreeBSD$
 .\"
-.Dd March 20, 2011
+.Dd May 31, 2011
 .Dt IFCONFIG 8
 .Os
 .Sh NAME
@@ -42,7 +42,7 @@
 .Op Fl n
 .Ar interface
 .Op Cm create
-.Op Ar address_family
+.Ar address_family
 .Oo
 .Ar address
 .Op Ar dest_address
@@ -165,8 +165,10 @@ and
 .Dq link .
 .\" and
 .\" .Dq ns .
-The default is
-.Dq inet .
+The default if available is
+.Dq inet
+or otherwise
+.Dq link .
 .Dq ether
 and
 .Dq lladdr

sbin/ifconfig/ifconfig.c

@@ -220,8 +220,10 @@ main(int argc, char *argv[])
 		ifindex = 0;
 		if (argc == 1) {
 			afp = af_getbyname(*argv);
-			if (afp == NULL)
+			if (afp == NULL) {
+				warnx("Address family '%s' unknown.", *argv);
 				usage();
+			}
 			if (afp->af_name != NULL)
 				argc--, argv++;
 			/* leave with afp non-zero */
@@ -484,7 +486,28 @@ ifconfig(int argc, char *const *argv, int iscreate, const struct afswtch *uafp)
 	int s;
 
 	strncpy(ifr.ifr_name, name, sizeof ifr.ifr_name);
-	afp = uafp != NULL ? uafp : af_getbyname("inet");
+	afp = NULL;
+	if (uafp != NULL)
+		afp = uafp;
+	/*
+	 * This is the historical "accident" allowing users to configure IPv4
+	 * addresses without the "inet" keyword which while a nice feature has
+	 * proven to complicate other things.  We cannot remove this but only
+	 * make sure we will never have a similar implicit default for IPv6 or
+	 * any other address familiy.  We need a fallback though for
+	 * ifconfig IF up/down etc. to work without INET support as people
+	 * never used ifconfig IF link up/down, etc. either.
+	 */
+#ifdef INET
+	if (afp == NULL && feature_present("inet"))
+		afp = af_getbyname("inet");
+#endif
+	if (afp == NULL)
+		afp = af_getbyname("link");
+	if (afp == NULL) {
+		warnx("Please specify an address_family.");
+		usage();
+	}
 top:
 	ifr.ifr_addr.sa_family =
 		afp->af_af == AF_LINK || afp->af_af == AF_UNSPEC ?