Fix #116 - zombie processes with posix driver

[tchaloupka]

waitpid must be called to avoid zombie processes.
See http://man7.org/linux/man-pages/man2/waitpid.2.html

[tchaloupka]

Another solution can be SA_NOCLDWAIT used with sigaction, but as we want to handle only known child processes, explicit call to waitpid seems more correct.

[tchaloupka]

CI errors seems unrelated

[PetarKirov]

Looks good to me. Is is possible to add a test?

[s-ludwig]

Needs to be rebased to account for the change in #114. I also think that in addition to calling waitpid, it also has to be used to control the loop within onSignal, because multiple signals can result in just a single signalfd_siginfo being reported by read. For that to work, waitpid would be called with a pid argument of -1 and then returns the exited PID, if any.

[tchaloupka]

Superseded by #122