Support all allowed channel prefixes

RFC 2812 specifies in §1.3 that “Channels names are strings (beginning
with a '&', '#', '+' or '!' character) of length up to fifty (50)
characters.”

We previously only allowed the first two prefix characters, or any other
one if the server overrides the list with ISUPPORT.

src/bridge/bridge.cpp

@@ -1160,7 +1160,7 @@ std::set<char> Bridge::get_chantypes(const std::string& hostname) const
 {
   IrcClient* irc = this->find_irc_client(hostname);
   if (!irc)
-    return {'#', '&'};
+    return {'&', '#', '+', '!'};
   return irc->get_chantypes();
 }