Provide ability to lock port rewiring.

The idea is to allow only a "single" frontend to rewire ports at
a certain point in time. Regarding a stream, frontend_lock is used
for example to allow only a single frontend to change the speed.
Unfortunately, frontend_lock cannot be used as the rewire functions
are not stream related.
Therefore a new port_rewiring_lock was introduced and used at
appropriate locations. When an arbitrary thread now holds the
frontend_lock and the port_rewiring_lock, it is safe that acquiring
a port ticket in functions like xine_get_current_frame() will
never block the thread.

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