When working on the Shepherd, I found that in the build containers 
processes don't get reaped by pid 1. See 
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=30637#29. This 
caused (and will cause) the Shepherd's tests to fail on some 
systems.

Our guile-builder script should handle SIGCHLD and then use 
waitpid to reap the child processes. Here's my attempt at a patch 
to do that.

I haven't been able to build anything with it because the computer 
I'm currently on is laughably slow. If someone else can check that 
you can still build with it I'd really appreciate it.