pidref: hookup PID_AUTOMATIC special pid_t value with PidRef

The PID_AUTOMATIC value is now properly recognized by the PidRef logic
too. This needed some massaging of header includes, to ensure pidref.h
can access process-util.h's definitions and vice versa.

src/basic/pidref.c

@@ -428,6 +428,10 @@ int pidref_wait_for_terminate(const PidRef *pidref, siginfo_t *ret) {
         }
 }
 
+bool pidref_is_automatic(const PidRef *pidref) {
+        return pidref && pid_is_automatic(pidref->pid);
+}
+
 static void pidref_hash_func(const PidRef *pidref, struct siphash *state) {
         siphash24_compress_typesafe(pidref->pid, state);
 }