Do not attempt to load the kernel module when checking if an interface exists.

This would cause pseudo network modules to be reloaded again when trying to
unload the first time if any cloned interfaces exist.

MFC after:	2 weeks

etc/network.subr

@@ -265,7 +265,7 @@ ipv6if()
 #	Returns 0 if the interface exists and 1 otherwise.
 ifexists()
 {
-	ifconfig $1 > /dev/null 2>&1
+	ifconfig -n $1 > /dev/null 2>&1
 }
 
 # ipv4_up if