gpg: default to AES-256.

gpg: default to AES-256.

* g10/main.h (DEFAULT_CIPHER_ALGO): Prefer AES256 by default.

--

It's 2017, and pretty much everyone has AES-256 available.  Symmetric
crypto is also rarely the bottleneck (asymmetric crypto is much more
expensive).  AES-256 provides some level of protection against
large-scale decryption efforts, and longer key lengths provide a hedge
against unforseen cryptanalysis.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>
(cherry picked from commit 73ff075204df09db5248170a049f06498cdbb7aa)

Gbp-Pq: Topic from-master
Gbp-Pq: Name gpg-default-to-AES-256.patch