dhcpv6-server: T2438: change systemd service type, validate config file
dhcpv6-server: T2438: change systemd service type, validate config file

The default of systemd services Type=simple isn't suitable for dhcpd and
other daemons.

- change service type to forking
- add RuntimeDirectory
- set paths to files in Environment
- set PIDFile
- validate config and lease file in ExecStartPre
- add -q to make dhcpd quiet and only log to syslog
- set Restart=always

vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x