crypto/openpgp: assorted cleanups

crypto/openpgp: assorted cleanups

1) Include Szabolcs Nagy's patch which adds serialisation for more
   signature subpackets.
2) Include Szabolcs Nagy's patch which adds functions for making DSA
   keys.
3) Make the random io.Reader an argument to the low-level signature
   functions rather than having them use crypto/rand.
4) Rename crypto/openpgp/error to crypto/openpgp/errors so that it
   doesn't clash with the new error type.

R=bradfitz, r
CC=golang-dev
https://golang.org/cl/5528044