clients: set routing search domain with nameservers when importing WireGuard profile

When the users configure a DNS server on the interface, they likely
want to use it, regardless whether there is a default route on the
device.

For that to work, add an explicit "~" search domain.

Otherwise, by default NetworkManager only adds the special search domain
only on devices that have a "best default route" (nm_ip_config_best_default_route_is).
But that only considers a best default route in the main table, and
WireGuard (with ipx-auto-default-route) adds the default route to a
separate table. The heuristic to determine best devices works not well
with policy routing, so explicitly add this search domain during import.

#405