include/inst: Add inst module for managing running instances of scripts

include/inst: Add inst module for managing running instances of scripts

This commit adds the inst module which provides functions for managing
instances of scripts:

 - The inst_start() function forks a function to the background
 - The inst_stop() function asks an instance to stop
 - The inst_list() function lists running instances
 - The inst_running() function tells an instance if it should stop

Internally, the module makes use of semaphores to list instances and
determine their state. Semaphores are kept in per-script directories
within $TOOLBOX_HOME/inst.