core: drop ambient capabilities in user manager

Ambient capabilities should not be passed implicitly to user
services. Dropping them does not affect the permitted and effective sets
which are important for the manager itself to operate.

(cherry picked from commit 963b6b906e)

src/core/main.c

@@ -2806,6 +2806,11 @@ int main(int argc, char *argv[]) {
                 /* clear the kernel timestamp, because we are not PID 1 */
                 kernel_timestamp = DUAL_TIMESTAMP_NULL;
 
+                /* Clear ambient capabilities, so services do not inherit them implicitly. Dropping them does
+                 * not affect the permitted and effective sets which are important for the manager itself to
+                 * operate. */
+                capability_ambient_set_apply(0, /* also_inherit= */ false);
+
                 if (mac_selinux_init() < 0) {
                         error_message = "Failed to initialize SELinux support";
                         goto finish;