Requests the passphrase with a popup.
As from GPGME 1.15.0, the loopback pinentry is functional when exporting secret keys. It works fine when the exact passphrase is provided. If it's a wrong passphrase, GPGME does not generate an ::Error, but the app crashes with 'free(): double free detected in tcache 2'.
Hence, this patch cannot be committed to master.
Status : dangerous
Result : works and works not
Reason : a wrong passphrase means a crash
Result : fails.
Reason : password callback is never called.
With default pinentry mode, the password is requested normally
and the private key is exported. But this can't be done on a web server.
Enclosed in #ifdef DEVTIME.
