Start services with clone3() and place in cgroup directly

This commit activates the use of clone3() for service_fork(), to allow
Linux to create the new process directly in the correct cgroup instead
of later moving it there -- much cheaper and less error prone.

To facilitate this a few new helper cgroup functions have been added and
two new configuration directives introduced: delegate and name:leafname.
The delegate option is for running, e.g., container runtimes that want
to create their own cgroup v2 structur, and the name:leafname allows a
user to change the name of the subgroup under user/system/init.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/helpers.c

@@ -463,7 +463,7 @@ void networking(int updown)
 				_exit(EX_OSERR);
 			_exit(WEXITSTATUS(rc));
 		}
-		cgroup_service("network", pid, NULL);
+		cgroup_service("network", pid, NULL, NULL, NULL);
 		print(pid > 0 ? 0 : 1, "%s network interfaces ...",
 		      updown ? "Bringing up" : "Taking down");