irclib: Filter out unsupported umodes before sending them to the server

Closes: Sf#3075891
Signed-off-by: James McCoy <vega.james@gmail.com>

src/irclib.py

@@ -928,9 +928,14 @@ class Irc(IrcCommandDispatcher):
         # Let's reset nicks in case we had to use a weird one.
         self.alternateNicks = conf.supybot.nick.alternates()[:]
         umodes = conf.supybot.protocols.irc.umodes()
+        supported = self.supported.get('umodes')
         if umodes:
-            if umodes[0] not in '+-':
-                umodes = '+' + umodes
+            addSub = '+'
+            if umodes[0] in '+-':
+                (addSub, umodes) = (umodes[0], umodes[1:])
+            if supported:
+                umodes = filter(lamda m: m in supported, umodes)
+            umodes = ''.join(addSub, umodes)
             log.info('Sending user modes to %s: %s', self.network, umodes)
             self.sendMsg(ircmsgs.mode(self.nick, umodes))
     do377 = do422 = do376