udev,sd_device: also save map from device ID to watch handle in /run/udev/watch

udev,sd_device: also save map from device ID to watch handle in /run/udev/watch

Previously, watch handle is saved in the udev databse. But in most cases,
the handle saved in the database is not updated. Especially, when udevd
is restarted, the inotify watch is restarted, but the database is not
updated.

Moreover, it is not necessary to save watch handle in the database, as
the handle is only take a effect during udevd is running, and the value
is meaningless when udevd is restarted.

So, this makes the opposite map from device ID to watch handle is saved
in /run/udev/watch as a symbolic link, and the handle not saved in the
database anymore.

Fixes #18525.