Now, the `sigaltstack` syscall is implemented and a alternate stack can
be defined.
A `signal` directory is created and the files for signals management are
splitted.
Features done:
*  Implement a signal queue for each process which contains all pending signals
*  Implement signal actions, a way to define a handler function for a specific signal
*  Implement signal masks, a way to block a signal from being delivered
*  Save the context and the SSE state during signal handlers