Access signalfd_siginfo from signal handler

[BenjaminSchaaf]

Posix signal handlers come with a lot of useful information, but the only thing currently made available is the signal that was raised. I'm currently dealing with processes and using SIGCHLD to catch when a process has terminated, but I can't get the process ID from the signal handler through the current api.

A quick fix would be just replacing the int with a signalfd_siginfo in the callback, but I'm not sure how that fits with the current design philosophy. Can't test but it shouldn't break Windows builds considering signals aren't implemented there.

[s-ludwig]

Just adding a comment regarding SIGCHLD - the actual information is not very useful due to signal coalescing. The porposed fix uses waitpid instead to get the status: #122