Add signal examples

[domfarolino]

We want examples demonstrating/do the following:

• That the process mask changes temporarily in signal handlers
  • And that "using sigprocmask inside the handler only affects what signals can arrive during the execution of the handler itself" (one of the last paras of https://www.gnu.org/software/libc/manual/html_node/Signals-in-Handler.html).
• Check the pending signals for a process in a signal handler via sigpending
• Show the coalescence of pending signals of the same type (this can be done by letting signals accumulate either in a slow handler (that lets the sigs accumulate) or by explicitly blocking signals of a certain type (letting sigs accumulate), and then unblocking the signals and showing how many times the handler is run (prob 1).
• sigprocmask vs pthread_sigmask
  • Demonstrate how setting a thread's signal mask doesn't change the containing process's signal mask, thus allowing threads to selectively choose which signals they can and cannot handle.