gpgsm: Allow ciphers AES192 and SERPENT256
* sm/gpgsm.c (main): Add AES192 cipher. Allow SERPENT256.
Note that currently released Libcgrypt versions miss OIDs for Serpent
and thus Serpent can only be used with tye forthcoming Libgcrypt
1.7.1.
(backport of master commit 5f9bd7a9e1ed4edcbb6c4e908d4bea5cd7dc9e68)