gpg: Make --export-ssh-key work for the primary key.

* g10/export.c (export_ssh_key): Also check the primary key.

If no suitable subkey was found for export, we now check whether the
primary key is suitable for export and export this one. Without this
change it was only possible to export the primary key by using the '!'
suffix in the key specification.

Also added a sample key for testing this.

• GnuPG-bug-id: T2957
• Signed-off-by: Werner Koch <wk@gnupg.org>