auth-dialog: always return found secrets for ExternalUiMode

Like the original stdout-based communication, always return any secrets
which were found.  If the secret doesn't need to be requeted from the
user, ensure "ShouldAsk" is FALSE.

This fixes a problem with GNOME Shell where both secrets were
stored by the user agent.  Since agents delete any saved secrets before
saving new secrets, this would delete the "IPSec secret" item, but since
that item was not included in the returned secrets, it wouldn't be
re-saved, and would be lost.