Thread scheduling simulator for a single processor system.

Scheduler uses a Round Robin algorithm implemented using a priority queue.
A semaphore is associated to each simulated thread in order to mimic preemption.

The state in which any thread is found within the simulation is one of the following:
NEW - threads that are being created, any new thread that has not yet been put in the thread queue
READY - threads in the thread queue waiting to be scheduled
RUNNING - one single thread in the system at a given moment, the one with the unlocked semaphore
WAITING - threads found in the IO waiting structure (the list vector)
TERMINATED - any thread that finishes running its handler function

In order for the simulator to work, the initial forked thread is assigned a negative priority. This makes sure that it is always at the end of the priority queue and is scheduled after all of the other threads have finished running.

A global thread structure list is maintained in order to keep track of all the active threads in the system. When reaching the TERMINATED state, a thread is deleted from the list.