systemd/knot-resolver.service.in tweaks

Mostly picked from !1377 (merged)

added manager label

resolved all threads

added 6 commits

• e8489616...f4c63abe - 3 commits from branch manager
• 20cd466e - manager: knot-resolver command
• 9035bbba - manager: update systemd service file with knot-resolver script and kresctl reload
• 2635762b - systemd/knot-resolver.service.in: generalize paths

Compare with previous version

unit:py3.6 surely fails due to 3.6 getting removed from CI image during !1400 (merged)

mentioned in commit 8a22dac1

merged