fix a problem creating and displaying connected status for ports not owned by ardour (e.g. system:....)

This was caused by using jack_port_get_connections() which will
not return the correct status for ports owned by another JACK
client. Because of the potential for deadlock by calling
jack_port_get_all_connections(), an extra argument was added
to several PortEngine:: API calls to specify whether the call
is in a process-callback context, which defaults to true.

The only place where false is passed is within the GlobalPortMatrix
when we need to determine whether two non-ardour ports are
connected.