* top.c (SIGJMP_BUF, SIGSETJMP, SIGLONGJMP): Update comments.

	* top.c (SIGJMP_BUF, SIGSETJMP, SIGLONGJMP): Update comments.
	(error_return, quit_return): Merge into catch_return pointer.
	(return_to_top_level): Update comment.  Longjmp to *catch_errors,
	and communicate reason to catch_errors via setjmp return value.
	(catch_errors): Always catch both quit and error, and if a catch
	wasn't requested by caller, throw it to the next catch_error.
	Replace dual longjmp buffer memcpy with single pointer change.
	Add FIXME for possibly adding new interface to tell caller what
	event was caught.  Add extensive comments.
	* defs.h (enum return_reason): Reserve 0 for use as initial
	setjmp() return value.
	(RETURN_MASK): New public macro to generate RETURN_MASK_* from
	enum return_reason.
	(RETURN_MASK_QUIT, RETURN_MASK_ERROR): Define using RETURN_MASK.