gpg-agent features of loopback-pinentry mode, preset_passphrase

[NIIBE Yutaka]

Hello,

While trying fix the issue 1928: https://bugs.gnupg.org/gnupg/issue1928

With GnuPG 2.1, the secret keys are under control of gpg-agent.

Invoking gpg with --passphrase (-file, -fd), the gpg frontend needs to
supply passphrase to gpg-agent.  I think that the feature of
loopback-pinentry mode and/or preset_passphrase could be used for
that.

However, those features are disabled as defaults.  Are there any
reason those feature could be disabled?

I think that loopback-pinentry mode should be always supported so that
--passphrase option of gpg can work well.
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