netplug: T5476: rewrite dhclient helper from Perl -> Python

There are two hooks called for bridge, ethernet and bond interfaces if the
link-state changes up -> down or down -> up.

The helpers are:
* /etc/netplug/linkdown.d/dhclient
* /etc/netplug/linkup.d/dhclient

As those helpers use Linux actions to start/restart the dhclient process in
Perl it's time to rewrite it. First goal is to get rid of all Perl code and the
second is that we now have a Proper Python library. Instead of checking if the
process is running the then restarting it without even systemd noticing
(yeah we might get two processes beeing alive) we should:

* Add a Python helper that can be used for both up and down (see man 8 netplugd
  FILES section)
* Query the VyOS CLI config if the interface in question has DHCP(v6)
  configured and is not disabled
* Add IPv6 DHCPv6 support

MAN page: https://linux.die.net/man/8/netplugd

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