* It's clearer what happens here: we try to remove as many matching
passwords as possible.
* Also rename secret_service_remove() to secret_service_clear()
* Rename secret_password_clear() which used to wipe password memory
to secret_password_wipe().
* The SecretService methods aren't used as frequently as the
password methods, and it's not really necessary to have a whole
bunch of extra varargs functions.
* Add varargs functions for building a GHashTable of attributes
