* cygthread.cc (cygthread::stub): Detect if thread function wants to release

itself here, to avoid a race.
(cygthread::release): Clear more stuff.  Add a diagnostic for an internal
error.
* cygthread.h (auto_release): New function.
* pinfo.h (pinfo::remember): Add an argument to denote whether child is
detached.
* fork.cc (fork_parent): Reflect change in arguments to pinfo::remember.
* pinfo.cc (_pinfo::exit): Signal exit more forcibly.
(proc_waiter): Use cygthread::auto_release to signify that cygthread::stub
should release the thread.  This should avoid a race.
(pinfo::alert_parent): Don't signify an error when wr_proc_pipe == NULL.
* sigproc.cc (proc_subproc): Add support for PROC_DETACHED_CHILD.
* sigproc.h: Ditto.
* spawn.cc (spawn_guts): Specify whether child is detached or not when calling
pinfo::remember.

winsup/cygwin/cygthread.h

@@ -32,6 +32,7 @@ class cygthread
   static DWORD WINAPI simplestub (VOID *);
   static DWORD main_thread_id;
   static const char * name (DWORD = 0);
+  void auto_release () {func = NULL;}
   void release (bool);
   cygthread (LPTHREAD_START_ROUTINE, LPVOID, const char *);
   cygthread () {};