network: keep dynamic configurations as possible as we can on reconfigure

network: keep dynamic configurations as possible as we can on reconfigure

E.g. when a .network file is updated, but DHCP setting is unchanged, it
is not necessary to drop acquired DHCP lease.
So, let's not stop DHCP client and friends in link_reconfigure_impl(),
but stop them later when we know they are not necessary anymore.

Still DHCP clients and friends are stopped and leases are dropped when
the explicit reconfiguration is requested