be-tarask language subtag registration form

[Frank Ellermann]

Michael Everson wrote:
 
> We should have two subtage I think.

Metoo, but it's not the job of the submitter to figure this 
out - apparently they think it would get them in mine-fields
they're not interested in.  The hypothetical spell-checker
could be configured to recognize 'tarask' and use "official"
without it (until a second variant is registered, allowing
smarter configurations per user).

Frank