manager: send an sd_notify() message informing the container manager when systemd's special UNIX signals become available

From the outside it's difficult to determine whether (and when) the PID1
inside a container supports systemd's more complete set of UNIX process
signals or not. Let's make this easier, and simply send a notification
message when we are ready.

src/core/manager.c

@@ -594,6 +594,17 @@ static int manager_setup_signals(Manager *m) {
         if (r < 0)
                 return r;
 
+        /* Report to supervisor that we now process the above signals. We report this as level "2", to
+         * indicate that we support more than sysvinit's signals (of course, sysvinit never sent this
+         * message, but conceptually it makes sense to consider level "1" to be equivalent to sysvinit's
+         * signal handling). Also, by setting this to "2" people looking for this hopefully won't
+         * misunderstand this as a boolean concept. Signal level 2 shall refer to the signals PID 1
+         * understands at the time of release of systemd v256, i.e. including basic SIGRTMIN+18 handling for
+         * memory pressure and stuff. When more signals are hooked up (or more SIGRTMIN+18 multiplex
+         * operations added, this level should be increased).  */
+        (void) sd_notify(/* unset_environment= */ false,
+                         "X_SYSTEMD_SIGNALS_LEVEL=2");
+
         if (MANAGER_IS_SYSTEM(m))
                 return enable_special_signals(m);