Do not compress passwords.

According to a forthcoming paper by Alfredo Pironti, OpenPGP compression
can reveal entropy levels. We thus disable compression.

Existing password stores can be reencrypted without compression using
the "--reencrypt" flag for "init".

Reported-by: Alfredo Pironti <alfredo.pironti@inria.fr>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>