Adds return values to the startup function

[XCompWiz]

The start function used to have return values to indicate success, which is used further down to control the start up of the monitor. Presently, I don't think that the monitor is started due to these returns being missing.

[jeffreykog]

The monitor is not starting because the -z check in mc_startmonitor is incorrect. It should be -n.

-z checks if the length of the given string is zero, thus meaning that if $CHECKSERVER is not defined, the monitor will start.
-n checks if the length of the given string is non-zero, which is what it should be.