* exceptions.cc (sigpacket::process): Move signal_exit processing into...

* exceptions.cc (sigpacket::process): Move signal_exit processing into...
(_cygtls::signal_exit): ...here.  Close my_readsig and comment on why.
* pinfo.cc (pinfo::exit): Move sigproc_terminate earlier.  Set exiting flag in
lock_process.
* sigproc.cc (my_readsig): Make global.
* sync.cc (muto::exiting_thread): Delete.
(muto::acquire): Delete #if 0'ed code.
* sync.h (muto::exiting_thread): Delete.
(set_exiting_thread): Ditto.
(lock_process::lock_process): Don't worry about setting the exiting thread
since it had no meaning.