gcr: Enable gnome2 keyring storage

It's the only one that sorta works currently. This really is
a technology preview however.
gcr: Fix invalid warning when sending a secret exchange

 * We checked the prefix of the sent data, but didn't correctly
   remove whitespace. In addition the check was inverted
 * Exposed by recent changes to glib, which changed the whitespace
   in the format a GKeyFile slightly
gcr: Add debug output to GcrSecretExchange