ipfw: use 'delete' instead of 'del' to avoid a warning on freebsd.

'del' is an abbreviation that happened to work because of substring matching
in earlier versions of ipfw, but apparently they're planning to remove the
substring matching eventually.  In any case, 'delete' has always worked, so
there's no downside to using that.

Reported by Ed Maste.

firewall.py

@@ -122,7 +122,7 @@ def do_ipfw(port, subnets):
 
     # cleanup any existing rules
     if ipfw_rule_exists(port):
-        ipfw('del', sport)
+        ipfw('delete', sport)
 
     while _oldctls:
         (name,oldval) = _oldctls.pop()