MINOR: systemd: Add SystemD's Protect*= options to the unit file

MINOR: systemd: Add SystemD's Protect*= options to the unit file

While the haproxy workers usually are running chrooted the master
process is not. This patch is a pretty safe defense in depth measure
to ensure haproxy cannot touch sensitive parts of the file system.

ProtectSystem takes non-boolean arguments in newer SystemD versions,
but setting those would leave older systems such as Ubuntu Xenial
unprotected. Distro maintainers and system administrators could
adapt the ProtectSystem value to the SystemD version they ship.