Fix bugs in update-game-score, on MS-Windows and elsewhere.

Fix bugs in update-game-score, on MS-Windows and elsewhere.

 lib-src/update-game-score.c (read_score): Try reading a character before
 probing the stream for EOF.  Initialize score->score to zero,
 before reading and accumulating the score.
 (read_scores): Fix logic that determines which value to return.
 Close the input stream when finished reading the scores (avoids
 failures in overwriting the file with a new one on MS-Windows,
 since a file that is open cannot be deleted).
 lib-src/ntlib.h (rename): Don't undefine.
 lib-src/ntlib.c (sys_rename): New function, needed for
 update-game-score.