handle SIGCHLD when running as child subreaper

[dqminh]

When running under child subreaper mode, it's useful for nsenter to be able to reap child processes. We have seen cases where spawned user processes were not reaped properly (nvm-sh/nvm#650)

I dont see how we can write a test case for this yet. Manual steps to reproduce this are:

# terminal 1
> docker pull soareschen/ubuntu-zsh
> docker run -it --rm --name test-nvm soareschen/ubuntu-zsh zsh
> source ~/.nvm/nvm.sh
> ps auxf # no defunct processes

# terminal 2
> docker exec -it test-nvm zsh
> source ~/.nvm/nvm.sh
> ps auxf # lots of defunct processes before this patch, and no defunct processes after

Thanks @soareschen for the test image

[vmarmol]

LGTM

[mrunalp]

Thanks, looking good. I will quickly test it out.

[mrunalp]

LGTM