open the score file at the start and then drop all setgid privileges
while keeping a (close-on-exec) file descriptor open to it. In order
to allow this the static data files have to be made world readable.
In addition a potential buffer overrun with corrupted score files is
avoided by more careful use of scanf (note that SCORE_SCANF_FMT is
defined alongside the definition of the relevant structure).
Submitted in PR 8015 by Joseph Myers <jsm28@cam.ac.uk>
