first implementation of a user secret agent.

However, NM's support for user secret agents appears to be abysmal.
Secrets are not being saved when the connection is created, so
you have to create the connection, then edit it and type in your
passwords again to get them saved, as NM doesn't call the agent's
SaveSecrets method on connection creation, but only on edits. In
addition, connections with agent stored secrets can not be activated
(not even from nm-applet - yes, kded was running with the secret
agent at the same time), GetSecrets is not being called. Simply
nothing happens (tested with Wlan WPA2-PSK). NM crashes when trying
to create a connection which uses 802-1x-TLS, while nm-applet doesn't
even allow to do that, as no private key file can be selected
(gave it just a quick try, maybe I missed something).

This fixes the issue that insecure Wlan connections could not be edited.
This fixes storage of 802-1x secrets systemwide.

TODO: test it with KWallet
TODO: test GSM, CDMA and PPOE
TODO: major code cleanup (remove all settingpersistence etc)
TODO: someone should talk to the NetworkManager guys about secret agent support
(event nm-applet stores everything in NM's systemwide storage)