Signed-off-by: Thomas Haller <thaller@redhat.com>

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.

The connectivity-checking code would generally result in
NMManager:state going CONNECTING -> CONNECTED_GLOBAL -> CONNECTED_SITE
in the case where the connectivity check failed. The brief incorrect
CONNECTED_GLOBAL is bad, because clients might see it and do the wrong
thing.

Instead, when we are ready to switch from CONNECTING to CONNECTED_*,
do a connectivity check first, and switch to either CONNECTED_SITE or
CONNECTED_GLOBAL based on the result of that.

Build and use NMConnectivity regardless of build options; if you build
without libsoup, NMConnectivity will just always report that you have
full connectivity (like it does when you build with libsoup but don't
enable connectivity checking).

Remove some unnecessary comments and some unnecessary code. Fix
indentation.

It's a singleton that always exists. Callers shouldn't have to
refcount it.

Also, move its cleanup code from dispose() to finalize(), and remove
some unnecessary checks in it.