* net/secrets.el: Autoload the widget functions.

* net/secrets.el: Autoload the widget functions.
(secrets-search-items, secrets-create-item)
(secrets-get-attributes, secrets-expand-item): Attributes will be
stored on the password database without leading ":", as all other
clients do as well.
(secrets-mode): Fix docstring.
(secrets-show-secrets): Provide it as autoloaded command only when
D-Bus support is available.  Check existence of Secret Service API.