* pkclist.c (select_algo_from_prefs): Make sure the scores can't
overflow when picking an algorithm (not a security issue since we
can't pick something not present in all preference lists, but we might
pick something that isn't scored first choice).
* pkclist.c (select_algo_from_prefs): Slightly improve the handling of
MD5 in preference lists.  Instead of replacing MD5 with SHA-1, just
remove MD5 from the list altogether, and let the next-highest ranked
algorithm be chosen.