* 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