Make use of the new API. Note that AddConnection2() covers all
functionality of AddConnection() and AddConnectionUnsaved(). Let's
only use one API for all.
There is a minor downside to this patch: now nmcli requires
libnm 1.20 API. Note that libnm's nm_client_add_connection2()
makes an effort to avoid AddConnection2() under the hood to
still work against older server versions. So, you can use nmcli
with libnm 1.20 to talk to older versions of NetworkManager.
But with this change nmcli strictly requires libnm 1.20. I think that is
sensible because commonly nmcli requires a libnm version that is as new
Also, the value of allowing nmcli to talk to older NetworkManager
versions is during package upgrade (where the daemon might not be
restarted). This is much less concern w.r.t. to updating the nmcli/libnm
combo, which is commonly packaged together.