gpg: Fail decryption for AES etc message w/o MDC.

* g10/mainproc.c (proc_encrypted): Fail for modern messages w/o MDC.

This change turns the missing MDC warning into an error if the message
has been encrypted using a cipher with a non-64 bit block length cipher
and it is not Twofish.

We can assume that such messages are created by code which should have
been able to create MDC packets. AES was introduced with 1.0.3 on
2000-09-18 shortly after MDC (1.0.2 on 2000-07-12). We need to
exclude Twofish because that might have been used before MDC.

• Signed-off-by: Werner Koch <wk@gnupg.org>