Reapply "Make battery module update on plugging/unplugging again (refs #2519)"

This reverts commit bb2c67ebad.

src/util/backlight_backend.cpp

@@ -1,4 +1,5 @@
 #include "util/backlight_backend.hpp"
+#include "util/udev_deleter.hpp"
 
 #include <fmt/core.h>
 #include <spdlog/spdlog.h>
@@ -29,22 +30,6 @@ class FileDescriptor {
   int fd_;
 };
 
-struct UdevDeleter {
-  void operator()(udev *ptr) { udev_unref(ptr); }
-};
-
-struct UdevDeviceDeleter {
-  void operator()(udev_device *ptr) { udev_device_unref(ptr); }
-};
-
-struct UdevEnumerateDeleter {
-  void operator()(udev_enumerate *ptr) { udev_enumerate_unref(ptr); }
-};
-
-struct UdevMonitorDeleter {
-  void operator()(udev_monitor *ptr) { udev_monitor_unref(ptr); }
-};
-
 void check_eq(int rc, int expected, const char *message = "eq, rc was: ") {
   if (rc != expected) {
     throw std::runtime_error(fmt::format(fmt::runtime(message), rc));