Add timeout to systemd script, stop child units and document absolute and relative folder names

[jankatins]

This is a bit of an overhaul of the systemd based sync:

• Document that one can use an absolute path as well (no code change,s just the eror message and the readme needed to be adjusted)
• Add a timeout to the grive run to prevent grive getting stuck and no one is noticing... (this happened freuently on my side and needed a killall grive a lot :-()
• Instruct the [email protected] unit to stop the timer and changes unit.
• Ignore some generated files
• some refactorings and dependency checking (I finally ran a shell linter on this file...) -> This should actually at some point run under set -e -u -o pipefail but I've jut started testing that...