revert setpgrp changes

Revert "Terminate pdns process after SIGINT" 5c663d99afcdd3c26d5744a47911840e16265b6d.
Revert "PDNS should always be the leader of the process group" 1a3bbc1bafea87169a1ed78f78a964ba06231b11.
Revert "Clean up after the pdns_server-instance" 6f8e772b634101a19106e23f20c5358357adf6d8.
Revert "Clean up after DynListener crash" 233f08a1de5508ca4ee67de9bc90045c2cd0cf63.
Revert "pdns: Kill all spawned processes" ee443da2d3ca2f831bbc2347088498890417ec13.
Revert "no systemd kill signals required, pdns_control take care of this" efd72fc50f71df4cd7036d960fa1814c15e4f230.
make sure the network-online.target is available
don't nuke the wrong service
Merge pull request #1688 from mind04/ungrp

revert setpgrp changes; fixes #1640, fixes #1671