udevadm: improve error output when a device is not specified or specified wrong

udevadm: improve error output when a device is not specified or specified wrong

udevadm would dump help() output, instead of printing a message about what is
wrong. That's just bad UX. Let's use a different message if the argument is
missing, and a different one if it is invalid.

Also, rework the code to separate the business logic from argument parsing.
Let's not use "default:" in switch statements. This way, the compiler will warn
us if we miss one of the cases.