* DevNotes: Add entry cgf-000018.

* DevNotes: Add entry cgf-000018.
* init.cc (dll_entry): Grab process lock before exiting to ensure that thread
doesn't exit before parent if parent is exiting.
* _cygtls.cc (_cygtls::call2): Revert previous 2012-12-21 change.
* miscfuncs.cc (thread_wrapper): Ditto.
* thread.cc (pthread::exit): Ditto.
* sigproc.cc (exit_thread): Ditto.
(wait_sig): Ditto.
* sync.cc (muto::release): Ditto.
* sync.h (muto::release): Ditto.
* sigproc.h (__SIGTHREADEXIT): Delete enum.
(exit_thread): Delete declaration.