(parity_16): New smaller parity table.

(des_check_parity): New function.
(des_fix_parity): Use parity_16.
(des_weak_p): New weak-key detection. Ignores parity bits, and
uses a hash table.
(des_set_key): Deleted parity checking code. Replaced old weak-key
detection code by a call to des_weak_p.

Rev: nettle/des.c:1.2
Rev: nettle/des.h:1.2