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>