* dcrt0.cc (reent_data): Reluctantly resurrect.

(__cygwin_user_data::impure_ptr): Ditto.
(_dll_crt0): Reluctantly initialize _impure_ptr here.
(initialize_main_tls): Eliminate local_clib initialization since it now happens
in init_thread.
* init.cc (dll_entry): Reluctantly remove code which set _impure_ptr to the
main thread's local_clib.
* perthread.h (reent_data): Remove obsolete declaration.
* sigproc.cc (proc_subproc): Add more debugging output.
(get_proc_lock): Ditto.
*dcrt0.cc (dll_crt0_1): Allocate argv[0] via malloc since main thread could
exit.

winsup/cygwin/perthread.h

@@ -12,9 +12,6 @@ details. */
 
 #define PTMAGIC 0x77366377
 
-struct _reent;
-extern struct _reent reent_data;
-
 #define PER_THREAD_FORK_CLEAR ((void *)UINT32_MAX)
 class per_thread
 {