We expect, that in many cases the strings for
_nm_utils_team_config_equal() are equal. E.g., when user updates
a connection only changing NMSettingIP4Config, but not touching the
team config. It makes sense to check for that first by doing plain
string comparison. In that case, we don't need any special Json
comparison.

Also, adjust the !WITH_JANSSON version, to return the same result
as WITH_JANSSON:
Before:
  WITH:    _nm_utils_team_config_equal (NULL, "", TRUE) => FALSE
  WITHOUT: _nm_utils_team_config_equal (NULL, "", TRUE) => TRUE

Optionally link libnm-core against jansson JSON library and use it to
validate and compare team configurations.

update_connection() may be called during startup when the bus watch
hasn't notified yet the presence (or absence) of the teamd service on
the bus. Try to obtain a connection to the service in order to
retrieve the current configuration.

Now that we support multiple VPNs active at the same time, the DNS
manager must be able to keep a list of all the VPN configurations and
give them a higher priority than other configurations.

Before this commit all the VPN configuration except one were
considered as normal configurations.

https://bugzilla.gnome.org/show_bug.cgi?id=764689

Fixes: 5f9d348c2019753712ff7444e3a8ae5ac3f38dda

When we want to preserve the default-route on cleanup, we must first
set it to assumed, before clearing it. Otherwise, NMDefaultRouteManager's
update() will delete the default route.

This is the oposite of the deconfigure case, where we first set it to
!has && !assumed, to force the route-manager to delete the route.