In particular, don't add trailing backslashes introduced by
`xautolockExtraOptions`. Systemd's unit file parser seems to have
gotten a bit stricter and with systemd 242, the trailing backslash
caused the next non-empty line to be ignored.
In that case, this was `[Section]`, so all subsequent settings were
mistakenly added to `[Service]`, causing them to be ignored entirely.
Simplify and fix this by using `concatStringsSep` to build a single
`ExecStart` line.
