This fixes a bunch of poorly formatted decls, marks some getters as
PURE, deletes some C-relevant bool hackery, and finally uses a
passed-in location rather than deducing a closely-related but not
necessarily the same location.

	* include/cpplib.h (cpp_get_otions, cpp_get_callbacks)
	(cpp_get_deps): Mark as PURE.
	* include/line-map.h (get_combined_adhoc_loc)
	(get_location_from_adhoc_loc, get_pure_location): Reformat decls.
	* internal.h (struct lexer_state): Clarify comment.
	* system.h: Remove now-unneeded bool hackery.
	* files.c (_cpp_find_file): Store LOC not highest_location.