rtsold: ignore SIGCHLD to reap the casper daemon

[bapt]

Summary

rtsold closes the casper channel after opening its services; the
kernel then kills the casper daemon with SIGKILL when the last
process descriptor is closed. As rtsold never waits for children,
the casper daemon lingers as a zombie for the process lifetime.

Commit bcdb6ba94d08 stopped unconditionally reparenting a pdfork(2)
child to init(8) when its last process descriptor is closed: the
parent is now expected to wait(2) or ignore SIGCHLD.

Ignore SIGCHLD so the kernel reaps exited children automatically.
Scripts are run by the rtsold.script service, so script status
handling is unaffected.

[bapt]

I am really not confident about this one, I tried to put all my understanding of the situation in the commit log, so reviewer can see what I understood

This is reproducible with rtsold -a -i with any kernel from after july I have not bissect to confirm 100% it appeared since @kib commit. bug adding this change fixes the issue here I don't see any zombie left.

[kib]

Are you stating that libcapser does not waitpid() on the resulting pid after pdfork()? If yes, the proper solution is to add PD_NOWAITPID to pdfork() call. This should fix it for all consumers.