rc: network.subr improve network6_getladdr()

In network6_getladdr() we are iterating over inet6 lines and are not
interested in any others.  So tell ifconfig to limit output to "inet6"
as much as possible.
This is probably a micro-optimisation but was noticed while looking
at other IPv6-related boot-time improvements.

MFC after:	1 week

libexec/rc/network.subr

@@ -1746,7 +1746,7 @@ network6_getladdr()
 	_if=$1
 	_flag=$2
 
-	${IFCONFIG_CMD} $_if 2>/dev/null | while read proto addr rest; do
+	${IFCONFIG_CMD} $_if inet6 2>/dev/null | while read proto addr rest; do
 		case "${proto}/${addr}/${_flag}/${rest}" in
 		inet6/fe80::*//*)
 			echo ${addr}