Edit systemd units

Unit files in systemd can be mended without modifying the actual unit file. This is useful to apply custom modifications to systemd services that will survive updates of the package, as system updates might reset the original unit file.

For instance, at the time of writting, issue haveged#41 was still present in Armbian 22.05.1 Bullseye. We can manually apply the fix to haveged service from commit haveged@159dcde2 with the following steps:

1. Edit haveged service file

   $ systemctl edit haveged.service

2. Add any modified or new parameters to the unit file

   ### Editing /etc/systemd/system/haveged.service.d/override.conf
   ### Anything between here and the comment below will become the new contents of the file
   
   [Service]
   SystemCallFilter=@system-service
   SystemCallFilter=~@mount
   SystemCallErrorNumber=EPERM
   
   ### Lines below this comment will be discarded
   

   Keep in mind to always specify the section of the parameters. {: .prompt-warning }