core: fix manager_get_unit_by_pid() special casing of manager PID

Previously, we'd hard map PID 1 to the manager scope unit. That's wrong
however when we are run in --user mode, as the PID 1 is outside of the
subtree we manage and the manager PID might be very differently. Correct
that by checking for getpid() rather than hardcoding 1.

src/core/cgroup.c

@@ -2183,7 +2183,7 @@ Unit *manager_get_unit_by_pid(Manager *m, pid_t pid) {
         if (pid <= 0)
                 return NULL;
 
-        if (pid == 1)
+        if (pid == getpid_cached())
                 return hashmap_get(m->units, SPECIAL_INIT_SCOPE);
 
         u = hashmap_get(m->watch_pids1, PID_TO_PTR(pid));