crypto: Set pinentry-mode to LOOPBACK
    
    We need to be able to supply the passphrase to gpgme. When using the
    default passphrase, the user is prompted and our callback isn't called.
    
    From the gpgme documentation:
    
     For GnuPG >= 2.1 this option is required to be set to
     GPGME_PINENTRY_MODE_LOOPBACK to enable the passphrase callback
     mechanism in GPGME through gpgme_set_passphrase_cb.