Labels must start with an alphanumeric character, may contain
alphanumeric characters or hyphens, but must not end with a hyphen.
We enforce those rules, but allow underscores wherever hyphens are
accepted, and allow alphanumeric ranges anywhere.

We relax the definition of "alphanumeric" to include Unicode characters
even though such inventory hostnames cannot be used in practice unless
an ansible_ssh_host is set for each of them.

We still don't enforce length restrictions—the fact that we have to
accept ranges makes it more complex, and it doesn't seem especially
worthwhile.