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.

test/apps/clone3/clone_exit_signal.c

@@ -0,0 +1,55 @@
+// SPDX-License-Identifier: MPL-2.0
+
+#define _GNU_SOURCE
+#include <err.h>
+#include <linux/sched.h>
+#include <signal.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/syscall.h>
+#include <sys/wait.h>
+
+static pid_t sys_clone3(struct clone_args *args)
+{
+	return syscall(SYS_clone3, args, sizeof(struct clone_args));
+}
+
+int child_exit_recv = 0;
+
+void sig_handler(int signal)
+{
+	printf("Received child exit signal\n");
+	child_exit_recv++;
+}
+
+int main(int argc, char *argv[])
+{
+	pid_t pid;
+	struct clone_args args = {
+		.exit_signal = SIGUSR2,
+	};
+
+	signal(SIGUSR2, sig_handler);
+
+	pid = sys_clone3(&args);
+	if (pid < 0)
+		err(EXIT_FAILURE, "Failed to create new process");
+
+	if (pid == 0) {
+		printf("Child process with pid %d\n", getpid());
+		exit(EXIT_SUCCESS);
+	}
+
+	/*
+	 * From the clone(2) manual:
+	 * If [the exit signal] is specified as anything other than SIGCHLD,
+	 * then the parent process must specify the __WALL or __WCLONE
+	 * options when waiting for the child with wait(2).
+	 */
+	waitpid(pid, NULL, __WALL);
+
+	if (child_exit_recv != 1)
+		errx(EXIT_FAILURE, "did not receive exit signal from child");
+
+	return 0;
+}