Fix SIGCHLD handling

[fingolfin]

SIGCHLD needs to be revised. There is an attempt to that in the code (see commits 48f0f1f and 3905afa), but it currently does not compile with stock GAP, and should probably reverted for the time being.

A proper fix requires changes to GAP itself. Note that other packages (e.g. libsing, which integrates Singular into GAP), will also need to deal with that. So this would be a useful thing anyway.

Max N., Reimer Behrends and Steve Linton already had a discussion about this at some point, so if anybody is interested in working on this, then talking to them might be a good starting point.

[rbehrends]

The io package should not install a SIGCHLD signal handler for HPC-GAP. HPC-GAP handles these signals in a separate thread and may therefore run waitpid() before io's signal handler can do so, causing io to hang indefinitely. Since the HPC-GAP handler should be able to handle this satisfactorily, there is also no need to have an additional signal handler.

[fingolfin]

This was now partially addressed, at least for non-HPC-GAP, by @ChrisJefferson's recent work. I think HPC-GAP compatibility still needs to be addressed.

[jdemeyer]

Cross-reference: gap-system/gap#3380