Consider allowing non-combined systemd service restarts

[onyxmaster]

Currently, the systemctl restart is invoked for all services that are being restarted. Sometimes, restarting all services in parallel is undesirable (for example to prevent overloading server). I think that adding a configuration option that prevents systemctl_combine being set to 1 when restarting services would be great. I would send a PR, but I wrote Perl 19 years ago so I think it might do more harm than good =)

P.S. Thanks for needrestart, it's a great tool!