sd-device,udev: tag must be a valid filename

sd-device,udev: tag must be a valid filename

All tags are managed under /run/udev/tags, and the directories there are
named with tags. Hence, each tag must be a valid filename.

This also makes all validity check moved to sd-device side, and
makes failure caused by setting invalid tags non-critical.

With this change, an empty string cannot be assigned to TAG=, hence the
test cases are adjusted.