core/device: Use DEVICE_NOT_FOUND instead of 0.

Use the enum name instead of the value zero. This is a noop change
as DEVICE_NOT_FOUND == 0.

src/boot/efi/boot.c

@@ -1916,7 +1916,7 @@ static ConfigEntry *config_entry_add_loader(
                 .device = device,
                 .loader = xstrdup(loader),
                 .id = xstrdup(id),
-                .sort_key = xstrdup(sort_key),
+                .sort_key = sort_key ? xstrdup(sort_key) : NULL,
                 .key = key,
                 .tries_done = UINTN_MAX,
                 .tries_left = UINTN_MAX,

src/core/device.c

@@ -885,7 +885,7 @@ static void device_remove_old_on_move(Manager *m, sd_device *dev) {
         if (!syspath_old)
                 return (void) log_oom();
 
-        device_update_found_by_sysfs(m, syspath_old, 0, DEVICE_FOUND_MASK);
+        device_update_found_by_sysfs(m, syspath_old, DEVICE_NOT_FOUND, DEVICE_FOUND_MASK);
 }
 
 static int device_dispatch_io(sd_device_monitor *monitor, sd_device *dev, void *userdata) {
@@ -924,7 +924,7 @@ static int device_dispatch_io(sd_device_monitor *monitor, sd_device *dev, void *
 
                 /* If we get notified that a device was removed by udev, then it's completely gone, hence
                  * unset all found bits */
-                device_update_found_by_sysfs(m, sysfs, 0, DEVICE_FOUND_MASK);
+                device_update_found_by_sysfs(m, sysfs, DEVICE_NOT_FOUND, DEVICE_FOUND_MASK);
 
         } else if (device_is_ready(dev)) {
 
@@ -941,7 +941,7 @@ static int device_dispatch_io(sd_device_monitor *monitor, sd_device *dev, void *
         } else
                 /* The device is nominally around, but not ready for us. Hence unset the udev bit, but leave
                  * the rest around. */
-                device_update_found_by_sysfs(m, sysfs, 0, DEVICE_FOUND_UDEV);
+                device_update_found_by_sysfs(m, sysfs, DEVICE_NOT_FOUND, DEVICE_FOUND_UDEV);
 
         return 0;
 }

src/core/mount.c

@@ -2070,7 +2070,7 @@ static int mount_process_proc_self_mountinfo(Manager *m) {
                         continue;
 
                 /* Let the device units know that the device is no longer mounted */
-                device_found_node(m, what, 0, DEVICE_FOUND_MOUNT);
+                device_found_node(m, what, DEVICE_NOT_FOUND, DEVICE_FOUND_MOUNT);
         }
 
         return 0;

src/core/swap.c

@@ -1249,7 +1249,7 @@ static int swap_process_proc_swaps(Manager *m) {
                         }
 
                         if (swap->what)
-                                device_found_node(m, swap->what, 0, DEVICE_FOUND_SWAP);
+                                device_found_node(m, swap->what, DEVICE_NOT_FOUND, DEVICE_FOUND_SWAP);
 
                 } else if (swap->just_activated) {