chapters/IO: Create signals section

[Alex-deVis]

Signals are an important interprocess communication (IPC) mechanism of the OS that is currently missing from the I/O chapter. It would be nice to have:

• a brief reading material describing the importance of sending, receiving, and ignoring signals, and the API.
• a task that builds on the previous examples, maybe something along the lines of sending SIGINT, ignoring SIGHUP, or attempting to handle SIGESGV.

[teodutu]

Signals should also be mentioned in the lecture slides.