x/crypto/openpgp: add ElGamal support when writing GPG keys.

When writing keys that contain ElGamal subkeys, the openpgp
package would previously return an error.

Change-Id: I1c487f4a9cfb92b40e99e9be6aa4ed5928e36dbe
Reviewed-on: https://go-review.googlesource.com/18254
Reviewed-by: Adam Langley <agl@golang.org>
2 files changed