* cygthread.h: Change 'avail' cygthread element to 'inuse' throughout.

* cygthread.h: Change 'avail' cygthread element to 'inuse' throughout.
* cygthread.cc: Ditto.
(cygthread::stub): Don't initialize already initialized events.
(cygthread::freerange): Don't create thread here.
(cygthread::cygthread): Create thread here.  Die if thread not created.
(cygthread::operator new): Simplify.  Just grab a thread structure from the
pool.  Don't try to start the thread.
(cygthread::terminate_thread): Don't close event handles.  Just reuse them.
Call MEM_RELEASE rather than MEM_DECOMMIT (from Joe Buehler).