OStatus: support @example.com/path/to/profile mentions as well as @profile@example.com (latter requires webfinger, former doesn't)

Plus misc warnings/notices cleanup in the submission path.

plugins/OStatus/classes/Ostatus_profile.php

@@ -698,7 +698,7 @@ class Ostatus_profile extends Memcached_DataObject
     {
         // Get the canonical feed URI and check it
         $discover = new FeedDiscovery();
-        if ($hints['feedurl']) {
+        if (isset($hints['feedurl'])) {
             $feeduri = $hints['feedurl'];
             $feeduri = $discover->discoverFromFeedURL($feeduri);
         } else {