add rtsol_flags.

MFC after:	1 week

etc/defaults/rc.conf

@@ -282,6 +282,7 @@ ipv6_router_flags=""		# Flags to IPv6 routing daemon.
 ipv6_default_interface="NO"	# Default output interface for scoped addrs.
 				# Now this works only for IPv6 link local
 				# multicast addrs.
+rtsol_flags=""			# Flags to IPv6 router solicitation.
 rtadvd_enable="NO"		# Set to YES to enable an IPv6 router
 				# advertisement daemon. If set to YES,
 				# this router becomes a possible candidate

etc/network.subr

@@ -407,7 +407,7 @@ network6_interface_setup()
 		sysctl net.inet6.ip6.accept_rtadv=1
 		set ${rtsol_interfaces}
 		ifconfig $1 up
-		rtsol $1
+		rtsol ${rtsol_flags} $1
 	fi
 
 	for i in $interfaces; do