cppfiles.c (struct _cpp_file): Rename once_only_next to next_file.

	* cppfiles.c (struct _cpp_file): Rename once_only_next to
	next_file.  Remove import and pragma_once, add once_only.
	(find_file): Add new file structures to the all_files list.
	(should_stack_file): Mark #import-ed files once-only, and
	don't stack them if the file has already been stacked.
	(_cp_mark_file_once_only): Simplify.
	* cpphash.h (struct cpp_reader): Rename once_only_files
	to all_files.  Rename saw_pragma_once to seen_once_only.
	(_cpp_mark_file_once_only): Update prototype.
	* cpplib.c (do_pragma_once): Update.
testsuite:
	* import1.c, import2.c: New tests.

From-SVN: r70106