Change service start up type from forking to exec

[rfc6919]

Type=forking leads to problems (eg: https://forum.greenbone.net/t/gsad-service-activating/12993/13) and is unnecessary since gsad has a --foreground option.

What:

Switch gsad.service unit file from Type=forking to Type=exec

Why:

systemd appears to lose track of the gsad process when using Type=forking and become unable to stop the service, which then prevents the service from being later restarted (because it was never actually stopped).

How:

I've been running this configuration across multiple systems for a while, and

1. it solves the service shutdown/startup problems
2. it doesn't appear to cause any new problems

Checklist:

• [ ] Tests N/A
• [ ] CHANGELOG Entry N/A
• [ ] Labels for ports to other branches N/A