* srfi-14.c (scm_char_set_diff_plus_intersection): wasn't correctly

	* srfi-14.c (scm_char_set_diff_plus_intersection): wasn't correctly
	accounting for the (char-set-union cs2...) in the spec.  i.e.,
	(char-set-diff+intersection a) -> copy-of-a, empty-set
	and the following are equivalent:
	(char-set-diff+intersection a (char-set #\a) (char-set #\b))
	(char-set-diff+intersection a (char-set #\a #\b))

	(scm_char_set_xor_x): disabled the side-effecting code, since it
	gives inconsistent results to scm_char_set_xor for the case
	(char-set-xor! a a a).

	(scm_char_set_diff_plus_intersection_x): added cs2 argument, since
	two arguments are compulsory in final spec.  also similar changes
	as for scm_char_set_diff_plus_intersection.
	* srfi-14.h (scm_char_set_diff_plus_intersection_x): added cs2.