process changes

process changes

This changes wait_reading_process_output to handle threads better.  It
introduces a wrapper for select that releases the global lock, and it
ensures that only a single thread can select a given file descriptor
at a time.

This also adds the thread-locking feature to processes.  By default a
process can only have its output accepted by the thread that created
it.  This can be changed using set-process-thread.  (If the thread
exits, the process is again available for waiting by any thread.)

Note that thread-signal will not currently interrupt a thread blocked
on select.  I'll fix this later.