libbacktrace: add DLLS as they are loaded

Patch from Björn Schäpers.

	* pecoff.c (struct dll_notification_data): Define.
	(LDR_DLL_NOTIFICATION): New typedef.
	(LDR_REGISTER_FUNCTION): New typedef.
	(struct dll_notification_context): Define.
	(dll_notification): New static function.
	(backtrace_initialize): Register DLL notification.