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

test/test-network/systemd-networkd-tests.py

@@ -6704,6 +6704,7 @@ class NetworkdDHCPClientTests(unittest.TestCase, Utilities):
                       '--dhcp-option=option6:ntp-server,[2600::ff]')
 
         networkctl_reload()
+        networkctl_reconfigure('veth99') # Release previously acquired lease and start new DHCPv6 handshake.
         self.wait_online('veth99:routable', 'veth-peer:routable')
 
         # checking address