udev-config: merge configured children max

Merge children_max from the per-source UdevConfig values before
defaulting it from system resources.

Reproducer:
  systemd-udevd --debug --children-max=1

Before:
  Set children_max to 32

Follow-up:
  04fa5c1580

src/udev/test-udev-manager.c

@@ -2,6 +2,7 @@
 
 #include "alloc-util.h"
 #include "tests.h"
+#include "udev-config.h"
 #include "udev-manager.h"
 
 TEST(devpath_conflict) {
@@ -35,4 +36,16 @@ TEST(worker_free_detaches_event) {
         ASSERT_NULL(event->worker);
 }
 
+TEST(manager_load_merges_children_max) {
+        _cleanup_(manager_freep) Manager *manager = ASSERT_PTR(manager_new());
+        char *argv[] = {
+                (char*) "systemd-udevd",
+                (char*) "--children-max=1",
+                NULL,
+        };
+
+        ASSERT_OK_POSITIVE(manager_load(manager, ELEMENTSOF(argv) - 1, argv));
+        ASSERT_EQ(manager->config.children_max, 1U);
+}
+
 DEFINE_TEST_MAIN(LOG_DEBUG);

src/udev/udev-config.c

@@ -281,6 +281,7 @@ static void manager_merge_config(Manager *manager) {
 
         manager_merge_config_log_level(manager);
         MERGE_NON_NEGATIVE(resolve_name_timing, RESOLVE_NAME_EARLY);
+        MERGE_NON_ZERO(children_max, 0);
         MERGE_NON_ZERO(exec_delay_usec, 0);
         MERGE_NON_ZERO(timeout_usec, DEFAULT_WORKER_TIMEOUT_USEC);
         MERGE_NON_ZERO(timeout_signal, SIGKILL);