gpg-agent: add new CACHE_MODE_EXPORT

* agent/agent.h: define CACHE_MODE_EXPORT
* agent/call-pinentry.c (agent_askpin, agent_get_passphrase): use "e/"
as the prefix for SETKEYINFO when in CACHE_MODE_EXPORT.
(agent_clear_passphrase): allow clearing the export cache.
* agent/command.c (cmd_clear_passphrase): add --mode=export.
(cmd_export_key): use CACHE_MODE_EXPORT.
* tests/openpgp/export.scm: no need to feed passphrases during export,
already cached.

We don't want secret keys to be able to be exported automatically
based on the same system passphrase cache used by standard decryption
or signing operations.

So we introduce a "export" cache mode which can be used by EXPORT_KEY.

I confess i don't fully understand the changes made to
tests/openpgp/export.scm -- i'm not sure why the passphrase is already
supplied in this case.

• Gnupg-Bug-Id: T4522
• Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>