gpg: Hard fail on a missing MDC even for legacy algorithms.

* g10/mainproc.c (proc_encrypted): Require an MDC or AEAD
* tests/openpgp/defs.scm (create-gpghome): Use --ignore-mdc-error to
allow testing with the current files.

(cherry picked from commit d1431901f0143cdc7af8d1a23387e0c6b5bb613f)

Resolved Conflicts:
g10/mainproc.c - Remove AEAD stuff.

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