Patch SHK_Taskmaster to require explicit initialization

* Require clients and server to call SHK_Taskmaster_initClient and
SHK_Taskmaster_initServer, respectively. This prevents listen servers
from initializating Taskmaster twice and running into race conditions.
* Add "Debug Mode" mission parameter and DEBUG flag in init.sqf.
Enabling either will cause only one mission target to be selected, to
make quick debugging sessions easy.