corecommands: block 'identify' from being used by command proxies such as networks.remote

This would have pretty bad unintentional consequences...

coremods/corecommands.py

@@ -16,6 +16,10 @@ from pylinkirc.log import log
 def _login(irc, source, username):
     """Internal function to process logins."""
 
+    if irc.is_internal_client(source):
+        irc.error("Cannot use 'identify' via a command proxy.")
+        return
+
     logindata = conf.conf['login'].get('accounts', {}).get(username, {})
     network_filter = logindata.get('networks')