Check if a service managed with update-rc.d in SysV systems is enabled

On a SysV system with services managed with the update-rc.d tool
(e.g on Debian), the service module, when executed with enabled=yes (or enabled=no),
it did not check if the service had already been enabled (or disabled), and was
always executing the enable (or disable) command, returning always changed=true.
This commit fixes this.