Try to detect file modification within the same second.

Try to detect file modification within the same second.
* buffer.h (struct buffer): New field modtime_size.
* buffer.c (reset_buffer): Initialize it.
* fileio.c (Finsert_file_contents, Fwrite_region): Set it.
(Fverify_visited_file_modtime): Check it.
(Fclear_visited_file_modtime, Fset_visited_file_modtime): Clear it.
(Fset_visited_file_modtime): Set (or clear) it.