* The holy grail: a function to export secret keys.
* GPGME will still invoke pinentry and gpg-agent as usual to authorise
  the export.
* Mostly similar to the two previous export functions for public keys
  except that it will return None if the result had a length of zero
  bytes.  Meaning that the difference between the specified pattern
  (if any) not matching available keys and an incorrect passphrase is
  not able to be determined from this function (or the underlying one
  for that matter).