gnetworkmonitornm: Consider NM_STATE_CONNECTED_SITE to be available
gnetworkmonitornm: Consider NM_STATE_CONNECTED_SITE to be available

`NM_STATE_CONNECTED_SITE` is documented to mean that a default route is
available, but that the internet connectivity check failed. A default
route being available is compatible with the documentation for
GNetworkMonitor:network-available, which should be true if the system
has a default route for at least one of IPv4 and IPv6.

https://developer.gnome.org/NetworkManager/stable/nm-dbus-types.html

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Fixes: #1788

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib