gpg: enable OpenPGP export of cleartext keys with comments

* g10/export.c (cleartext_secret_key_to_openpgp): ignore trailing
sublists in private-key S-expression.

When gpg-agent learns about a private key from its ssh-agent
interface, it stores its S-expression with the comment attached. The
export mechanism for OpenPGP keys already in cleartext was too brittle
because it would choke on these comments. This change lets it ignore
any additional trailing sublists.

• Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>
• Gnupg-Bug-Id: T4490