Set the default STATUSMSG to @ instead of @+ if it's missing

This fixes two issues:

- IRCNet doesn't have STATUSMSG, but it supports +channels, and
  including + in the default value meant processing those incorrectly

- The "bahamut hack", for old servers that support but don't advertise
  STATUSMSG, didn't work since ischannel_func doesn't use the default.

The choice of @ intentionally leaves out support for other STATUSMSG
(for example, AzzurraNet's bahamut 1.4 fork seemed to support + and % in
any order, contradicting the comment in the code).

I think this is a decent tradeoff, given how those servers are uncommon
and relying on +# or %# is even less common than @#.

Fixes #531

src/irc/core/irc-servers.c

@@ -89,8 +89,10 @@ static int ischannel_func(SERVER_REC *server, const char *data)
 		chantypes = "#&!+"; /* normal, local, secure, modeless */
 
 	statusmsg = g_hash_table_lookup(irc_server->isupport, "statusmsg");
-	if (statusmsg != NULL)
-		data += strspn(data, statusmsg);
+	if (statusmsg == NULL)
+		statusmsg = "@";
+
+	data += strspn(data, statusmsg);
 
 	/* strchr(3) considers the trailing NUL as part of the string, make sure
 	 * we didn't advance too much. */