log: honour the kernel's quiet cmdline argument

It was forgotten in b1e90ec515

See https://bugs.freedesktop.org/show_bug.cgi?id=79582

src/shared/log.c

@@ -878,6 +878,9 @@ void log_parse_environment(void) {
                         if (l == 5 && startswith(w, "debug")) {
                                 log_set_max_level(LOG_DEBUG);
                                 break;
+                        } else if (l == 5 && startswith(w, "quiet")) {
+                                log_set_max_level(LOG_WARNING);
+                                break;
                         }
                 }
         }