Handle signals

[Ahmedalshafy]

handling Ctlr-c, Ctlr-d and Ctlr-\ signals

[Pastifier]

Signals:

In main() (interactive mode):

• catch SIGINT and SIGQUIT
• SIGQUIT is ignored and does nothing. You can make it write "\b\b \b\b" if you want.
• SIGINT outputs a newline and redisplays the prompt.

  Right before fork():

• SIGINT and SIGQUIT are ignored. Do that by defining different signal handlers.

  Inside the fork():

• SIGINT and SIGQUIT are returned to their default behaviours (SIGDFLT)

  After the wait() in interpret():

• They're returned to their original values.
• You can do that in the main()'s while loop if you want, but it gives me an uneasy feeling, so be careful. If anything unexpected happens, tell me.