crypto/openpgp: better handling of keyrings.

crypto/openpgp: better handling of keyrings.

  * Accept armored private key blocks
  * If an armored block is missing, return an InvalidArgumentError,
    rather than ignoring it.
  * If every key in a block is skipped due to being unsupported,
    return the last unsupported error.
  * Include the numeric type of unsupported public keys.
  * Don't assume that the self-signature comes immediately after the
    user id packet.

R=bradfitzgo
CC=golang-dev
https://golang.org/cl/4434048