Make frame configurations start with a distinctive symbol.

Make frame configurations start with a distinctive symbol.
	* frame.el (current-frame-configuration): Return a list as before,
	but starting with `frame-configuration'.
	(set-frame-configuration): Check that CONFIGURATION is a list
	starting with `frame-configuration', strip it off, and do as before.
	(frame-configuration-p): New function.
	* register.el (jump-to-register): Use frame-configuration-p,
	instead of catching an error in set-frame-configuration.  Really now.