Plugin and daemon mode updates.

Add 'plugin' to the list of log masks parsed.

Fix and issue with daemon mode interacting with the plugin
callback system.  Daemon mode now uses the double-fork() method
to fully detach from the calling terminal.  Plugin callback
setup is now after the CreateDaemon() call.  Also fix a file
handle typo in plugins.c which may be related.

Add a couple debug messages for the plugin callback discovery.

I'll commit an updated binary after a final test on the Pi.