Respect user-defined exit signal in clone() and clone3()

When calling clone() and clone3(), the user is allowed to specify a
signal to be sent to the parent process on exit. Respect this value by
storing it in the Process struct and sending the signal on exit.

Add a test as well to verify that the signal is properly delivered to
the parent.

kernel/src/process/clone.rs

@@ -345,6 +345,10 @@ fn clone_child_process(
         process_builder.build()?
     };
 
+    if let Some(sig) = clone_args.exit_signal {
+        child.set_exit_signal(sig);
+    };
+
     // Deals with clone flags
     let child_thread = thread_table::get_thread(child_tid).unwrap();
     let child_posix_thread = child_thread.as_posix_thread().unwrap();