openpgp: support 3DES encrypted private keys.

It appears that CentOS (and so I presume Fedora/RHEL too) default to
encrypting private keys with 3DES so it looks like we need to support
that.

Fixes golang/go#3940.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/6446121
1 file changed