* dcrt0.cc (atexit_lock): Delete.

* dcrt0.cc (atexit_lock): Delete.
(cygwin_exit): Remove atexit lock.
(cygwin_atexit): Ditto.  Rename parameter to match newlib.  Call __cxa_atexit
when invoked by a registered DLL.
* dll_init.cc (remove_dll_atexit): Delete.
(dll_list::find): New function.
(dll_list::detach): Use dll_list::find to find dll associated with return
address.  Use __cxa_finalize to run atexit functions associated with the dll.
(cygwin_detach_dll): Don't assume that HANDLE == void *.
* dll_init.h (dll_list::find): Declare.
(__cxa_atexit): Ditto.
(__cxa_finalize): Ditto.
* init.cc (dll_entry): Clarify comment.