crypto/openpgp: add support for symmetrically encrypting files.
This mostly adds the infrastructure for writing various forms of
packets as well as reading them. Adding symmetric encryption support
was simply an easy motivation.
There's also one brown-paper-bag fix in here. Previously I had the
conditional for the MDC hash check backwards: the code was checking
that the hash was *incorrect*. This was neatly counteracted by another
bug: it was hashing the ciphertext of the OCFB prefix, not the
plaintext.
R=bradfitz
CC=golang-dev
https://golang.org/cl/
4564046