Adding bridge between protocol handlers (ports)

This is required to bridge traffic beween non-mISDN handlers,
such as GSM, SIP and voice box. Also it bridges traffic between
mISDN handlers and non-mISDN handlers. It is the fundamental step
to get rid of mISDN (loop interface) for non-mISDN handlers.
This is required to bridge audio e.g. between SIP and GSM without
using mISDN. There will be no limitations on 'b-channels' anymore.

Still GSM and SIP requires mISDN, but this will be changed later.

With that bridge I cleaned up some code and also removed the
MESSAGE_DATA, which is not required anymore.