gpg: Make OCB mode compliant in de-vs mode.

* common/compliance.c (gnupg_cipher_is_compliant): Support OCB for
gpg.
(gnupg_cipher_is_allowed): Ditto.
* g10/mainproc.c (proc_encrypted): Determine cipher mode and pass it
for the is_compliant test.

This patch also switches from just assuming CFB mode to passing the
correct mode to gnupg_cipher_is_compliant. Except for the legacy EAX
mode we only have these two modes and thus it does not really matter.
But correcting this is more future proof.

This was found while fixing

• GnuPG-bug-id: T7804