Make package scriptlets more universal

[iavael]

Package scriplets may fail on some system for various reasons like:

• restricted systemctl commands on systems like fedora silverblue
• lack of systemd overall on systems like devuan

So I made command invocation allowed to fail to make package scriptlets more robust in such edge cases. And also removed unnecessary subshell invocation.

[iavael]

@TheAssassin I'll take a deeper look during this week because I have some issues with broadband internet connection now.

[TheAssassin]

Thanks, I already left some comments.

[iavael]

Ok, I've finished all adjustments. Ready for review.

[iavael]

@TheAssassin in case, if message about adjustments had been finished got lost among notifications, humbly remind you about my PR being ready for review.