Prevent zombie processes in multi-process mode. (#36329)

This introduces a process manager that holds for each process a
"lifeline": this is the receiving end of a ipc channel that is not used
to send anything, but only to monitor the process presence. We turn that
ipc receiver into a crossbeam one to integrate the monitoring into the
constellation run loop. The sender side is made part of the initial
"UnprivilegedContent" data structure sent to the new process, both for
content and for service worker processes.
When a process dies we currently wait() on it to let the OS do a clean
shutdown.

Signed-off-by: webbeef <me@webbeef.org>

components/constellation/lib.rs

@@ -12,6 +12,7 @@ mod constellation;
 mod event_loop;
 mod logging;
 mod pipeline;
+mod process_manager;
 mod sandboxing;
 mod serviceworker;
 mod session_history;