cygserver: raise number of worker threads on demand

cygserver: raise number of worker threads on demand

The number of threads in the worker pool is fixed so far.  This is a
problem in XSI IPC scenarions with an unknown number of consumers.
It doesn't make sense to make the pool very big for a start, but when
the need arises, we need to make sure we can serve the request even if
all other worker threads are in a wait state.

This patch changes threaded_queue to just add another worker thread
if all current workers are busy.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>