Clean up redundancies between string.h and strings.h.

Clean up redundancies between string.h and strings.h.

	* string/string.h [__USE_MISC]: Include strings.h.
	(__bzero, bcmp, bcopy, bzero, index, rindex)
	(strcasecmp, strncasecmp, strcasecmp_l, strncasecmp_l)
	(ffs, ffsl, ffsll): Don't declare.

	* string/strings.h: Do not suppress the file if string.h has
	already been included.
	(bcmp, bcopy, bzero, strcasecmp, strncasecmp): Add __nonnull
	annotations.
	(index, rindex): Define inline forwarders even if
	__CORRECT_ISO_CPP_STRING_H_PROTO is defined.
	(ffs): Use __attribute_const__.
	(ffsl, ffsll): Declare here.
	(strcasecmp_l, strncasecmp_l): Correct comments; these functions
	have now been standardized.

	* include/string.h (__bzero): Declare here.