openpgp: support 3DES encrypted private keys.
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 issue 3940.

R=golang-dev, r
CC=golang-dev
http://codereview.appspot.com/6446121

cmars/sshlurp-go.crypto,isislovecruft/go.crypto,cmars/sshlurp-go.crypto,polydawn-ports/go.crypto,xman1101/terminal,cmars/go.crypto,tonnerre/golang-go.crypto,dizzyd/go.crypto,dizzyd/go.crypto,tonnerre/golang-go.crypto,polydawn-ports/go.crypto,drhax9908/go.crypto,isislovecruft/go.crypto,drhax9908/go.crypto,xman1101/terminal,cmars/go.crypto