core: split up manager_get_unit_by_pid()

Let's move the actual cgroup part of it into a new separate function
manager_get_unit_by_pid_cgroup(), and then make
manager_get_unit_by_pid() just a wrapper that also checks the two pid
hashmaps.

Then, let's make sure the various calls that want to deliver events to
the owners of a PID check both hashmaps and the cgroup and deliver the
event to *each* of them. OTOH make sure bus calls like GetUnitByPID()
continue to check the PID hashmaps first and the cgroup only as
fallback.

src/core/manager.c

@@ -1585,7 +1585,7 @@ static int manager_dispatch_notify_fd(sd_event_source *source, int fd, uint32_t
 
                 /* Notify every unit that might be interested, but try
                  * to avoid notifying the same one multiple times. */
-                u1 = manager_get_unit_by_pid(m, ucred->pid);
+                u1 = manager_get_unit_by_pid_cgroup(m, ucred->pid);
                 if (u1) {
                         manager_invoke_notify_message(m, u1, ucred->pid, buf, n, fds);
                         found = true;
@@ -1663,7 +1663,7 @@ static int manager_dispatch_sigchld(Manager *m) {
 
                         /* And now figure out the unit this belongs
                          * to, it might be multiple... */
-                        u1 = manager_get_unit_by_pid(m, si.si_pid);
+                        u1 = manager_get_unit_by_pid_cgroup(m, si.si_pid);
                         if (u1)
                                 invoke_sigchld_event(m, u1, &si);
                         u2 = hashmap_get(m->watch_pids1, PID_TO_PTR(si.si_pid));