bug#53013: 29.0.50; xwidget-webkit: SIGCHLD not being caught after xwidget-webkit buffer killed

[Akira Kyle]

In GNU Emacs 29.0.50 (build 1, aarch64-unknown-linux-gnu, GTK+ Version 3.24.30, 
cairo version 1.16.0)
Repository revision: ab5ee3e29e916d4009b301841e9780aad564a6a0
Repository branch: master
System Description: NixOS 22.05 (Quokka)

Configured using:
 'configure
 --prefix=/nix/store/0fiqa453abl4c1c28g0baqs9s274s7ff-emacs-pgtkgcc-20220103.0
 --disable-build-details --with-modules --with-x-toolkit=gtk3
 --with-cairo --with-xwidgets --with-native-compilation --with-pgtk'

Also tested on debian under GNOME (wayland) with the same configuration.

Steps to reproduce in emacs -Q:
- M-x xwidget-webkit-browse-url RET http://xkcd.com RET
- C-x k RET
- M-: (start-process "my-process" "foo" "echo" "hi")

No "Process my-process finished" line appears in buffer "foo" indicating
that emacs didn't catch SIGCHLD.

I suspect that a call to catch_child_signal might also be needed in
kill_xwidget so emacs reinstalls its signal handler.