Throughout use myself->ppid_handle rather than parent_alive.

* child_info.h (child_info): Eliminate parent_alive.
* dcrt0.cc (dll_crt0_1): Call fork_init for debugging pid creation.
* fork.cc (fork_child): Reflect change to fixup_mmaps_after_fork arguments.
(slow_pid_reuse): New function to grab last 'n' pids to prevent pid reuse.
(fork_parent): Move last_fork_proc into slow_pid_reuse.  Handle fork_pids
debugging.  Eliminate unnecessary call to set_child_mmap_ptr.
(fork_init): New debugging function.
* mmap.cc (fixup_mmaps_after_fork): Renamed from recreate_mmaps_after_fork.
Rely on copied data after a fork.
(set_child_mmap_ptr): Eliminate.
* pinfo.h (_pinfo): Eliminate parent_alive, mmap_ptr and reflect above changes.
* spawn.cc (spawn_guts): Eliminate vestiges of "old way" of sending new hProc
to parent process.

winsup/cygwin/child_info.h

@@ -36,7 +36,6 @@ public:
   HANDLE subproc_ready;	// used for synchronization with parent
   HANDLE shared_h;
   HANDLE console_h;
-  HANDLE parent_alive;	// handle of thread used to track children
   HANDLE parent;
   HANDLE pppid_handle;
   init_cygheap *cygheap;