networkd: stop clients when networkd shuts down (#12463)

networkd: stop clients when networkd shuts down (#12463)

We not stopping the clients when networkd stops. They
should shut down cleanly and then we need to clean the DS.

One of requirements to implement
https://github.com/systemd/systemd/issues/10820.

```
^CBus bus-api-network: changing state RUNNING → CLOSED
DHCP SERVER: UNREF
DHCP SERVER: STOPPED
DHCP CLIENT (0x60943df0): STOPPED
veth-test: DHCP lease lost
veth-test: Removing address 192.168.5.31
NDISC: Stopping IPv6 Router Solicitation client
DHCP CLIENT (0x0): FREE
==24308==
==24308== HEAP SUMMARY:
==24308==     in use at exit: 8,192 bytes in 2 blocks
==24308==   total heap usage: 4,230 allocs, 4,228 frees, 1,209,732 bytes allocated
==24308==
==24308== LEAK SUMMARY:
==24308==    definitely lost: 0 bytes in 0 blocks
==24308==    indirectly lost: 0 bytes in 0 blocks
==24308==      possibly lost: 0 bytes in 0 blocks
==24308==    still reachable: 8,192 bytes in 2 blocks
==24308==         suppressed: 0 bytes in 0 blocks
==24308== Rerun with --leak-check=full to see details of leaked memory
==24308==
==24308== For lists of detected and suppressed errors, rerun with: -s
==24308== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
==24308== could not unlink /tmp/vgdb-pipe-from-vgdb-to-24308-by-sus-on-Zeus
==24308== could not unlink /tmp/vgdb-pipe-to-vgdb-from-24308-by-sus-on-Zeus
==24308== could not unlink /tmp/vgdb-pipe-shared-mem-vgdb-24308-by-sus-on-Zeus

```