add label to enum syntax

It occurred to me in the dead of the night that instead of:
   ALTER TYPE enumtype ADD 'newlabel'


it might be better to have:
   ALTER TYPE enumtype ADD LABEL 'newlabel'


That way if we later wanted to support some other sort of ADD operation 
on types we would be able to more easily. LABEL is already a keyword, so 
it should be pretty minimally invasive to make this change, and if we 
want to do it now is the time.

Thoughts?

cheers

andrew