Fix miscellaneous update-game-score bugs.

Fix miscellaneous update-game-score bugs.

* configure.ac (difftime): Remove.
* lib/update-game-score.c (difftime) [!HAVE_DIFFTIME]: Remove.
(read_score) [HAVE_GETDELIM]: Don't access uninitialized storage.
(read_scores, write_scores): Check for fclose failure.
(write_scores): Use fchmod, not chmod, to avoid a race.
(lock_file): Fix test for out-of-date lock file; it was reversed.
Use ordinary subtraction rather than difftime; since we're already
assuming POSIX we don't need to worry about the possibility of
time_t being a magic cookie.