NM_STATE_CONNECTED_SITE doesn't distinguish between "behind a captive
portal" and "limited network connectivity" (ie, connected to a router
that has lost its upstream connection). Add a new NMManager
:connectivity property to provide this information.

Also add a CheckConnectivity method, which can be used to force NM to
re-check the connectivity state, which could be called by a client
after it completed a portal login, or fixed a network problem.