Arrange for Fy_or_n_p to put off switch-frame events.

Arrange for Fy_or_n_p to put off switch-frame events.
	* lread.c (read_filtered_char): New function, which contains the
	code which used to be in Fread_char, Fread_event, and
	Fread_char_exclusive; there was a lot of common code.
	(Fread_char, Fread_event, Fread_char_exclusive): Rewrite in terms
	of read_filtered_char.
	* lisp.h (read_filtered_char): Declare this extern here.
	* fns.c (Fy_or_n_p): Call read_filtered_char, arranging to delay
	switch-frame events.