Carbons: Also forward messages sent to bare JIDs

Don't ignore messages sent to bare JIDs, but forward them to all
carbon-copy-enabled resources that don't have the highest priority.
Carbons: Handle unavailable resource like bare JID

As the session manager handles messages sent to unavailable resources
just like messages sent to bare JIDs, mod_carboncopy must do that, too.
That is, forward them only to those carbon-copy-enabled resources that
don't have a top priority, in order to avoid duplicates.
Merge pull request #161 from weiss/fix-carbons

Let mod_carboncopy take care of messages sent to bare/unavailable JIDs