Remove signal context object from signal source component list (_signal_queue)
before destruction, otherwise we get a dangling pointer.
On native hardware for base-nova, the signal source thread triggered page
faults in the Signal_source_component::wait_for_signal() method when the signal
context got freed up in Signal_session_component::free_context but was still
enqueued in Signal_source_component::_signal_queue.
Fixes#600
Properly account signal count in the explicit-reply path (when source-
client gets immediately unblocked by 'signal_session_component::submit').
This patch prevents the delivery of superfluous signals with num == 0.
Norman Feske
Alexander Boettcher
Genode Labs