Allow user-defined service restart parameter.

This allows, for example, on CentOS/Debian/Ubuntu us to use /etc/init.d/haproxy reload to restart haproxy when config changes, which:

- includes syntax checks
- restarts without interruption to service

Without this it's easy to push out a option with, say a typo in it, which will cause haproxy to be left in a stopped state with broken config, because /etc/init.d/haproxy restart stops haproxy then starts it.

An alternative way to achieve this would be some logic to set an appropriate restart command for each distro, but I shied away from this, because it would presumably have to take into account things like Fedora's move to systemd.