(SCORE_FILE_PREFIX): Delete.

(SCORE_FILE_PREFIX): Delete.
(main): New argument -d, for specifying directory.
(usage): Document.
(get_user_id): Compute
(get_home_dir): Deleted.
(get_prefix): New function, taken from main.
(main): Check whether or not we are running setuid.  Move prefix
computation to get_prefix.  Don't call getpwent; we don't need to
any more.  Instead, move it to get_user_id().