g10: Export cleartext keys as cleartext
* g10/export.c (do_export_stream): If a key is stored by the agent in cleartext, then try to export it as cleartext. * tests/openpgp/export.test: For secret keys that are stored in cleartext, test should try to export without pinentry interaction.
This restores the behavior of GnuPG 2.0 and 1.4 when exporting
passphraseless secret keys, and fixes the test suite accordingly.