Add generic debug flag to MaxScale

For debugging purposes you may want to turn on or off certain behaviour.

Add handling for a generic --debug=arg1,arg2,arg3 startup flag using which that can be managed.

Initially support for --debug=disable-module-unloading, using which the unloading of modules at process shutdown can be disabled, should be added.

The arguments given to the --debug flag for enabling or disabling certain behaviour will always be of the format enable-X and disable-X.