* cipher/cipher-internal.h (gcry_cipher_handle): Add
'marks.allow_weak_key' flag.
* cipher/cipher.c (cipher_setkey): Do not handle weak key as error when
weak keys are allowed.
(cipher_reset): Preserve 'marks.allow_weak_key' flag on object reset.
(_gcry_cipher_ctl): Add handling for GCRYCTL_SET_ALLOW_WEAK_KEY.
* src/gcrypt.h.in (gcry_ctl_cmds): Add GCRYCTL_SET_ALLOW_WEAK_KEY.
* tests/basic.c (check_ecb_cipher): Add tests for weak key errors and
for GCRYCTL_SET_ALLOW_WEAK_KEY.