Fix a race with verify-visited-file-modtime.

Fix a race with verify-visited-file-modtime.

Since at least 1991 Emacs has ignored an mtime difference of no
more than one second, but my guess is that this was to work around
file system bugs that were fixed long ago.  Since the race is
causing problems now, let's remove that code.
* fileio.c (Fverify_visited_file_modtime): Do not accept a file
whose time stamp is off by no more than a second.  Insist that the
file time stamps match exactly.

Fixes: debbugs:12863