process-util: add simple wrapper around PR_SET_CHILD_SUBREAPER

Let's a simple helper that knows how to deal with PID == 1.

src/nspawn/nspawn.c

@@ -5826,8 +5826,9 @@ static int run(int argc, char *argv[]) {
 
         assert_se(sigprocmask_many(SIG_BLOCK, NULL, SIGCHLD, SIGWINCH, SIGTERM, SIGINT, SIGRTMIN+18, -1) >= 0);
 
-        if (prctl(PR_SET_CHILD_SUBREAPER, 1, 0, 0, 0) < 0) {
-                r = log_error_errno(errno, "Failed to become subreaper: %m");
+        r = make_reaper_process(true);
+        if (r < 0) {
+                log_error_errno(r, "Failed to become subreaper: %m");
                 goto finish;
         }