pdns: Kill all spawned processes

pdns: Kill all spawned processes

Pipe backend spawned processes can still be alive after the pdns_server-instance was killed.
If the children remain alive, sockets will not be released, and the pdns_server-instance can't be respawned.

To be sure that all the spawned processes are killed, we must kill by using the process GID.
To be able to do the soft reloads, pdns_server-instance is declared as process group lider.

Also, all forced kills must kill by GID, not by PID, so all the processes would properly die.

Signed-off-by: Igor Maravic <igor@spotify.com>
Signed-off-by: Alexey Lapitsky <alexey@spotify.com>