Add tab-completion for ALTER TABLE ADD NOT NULL

Hi,

psql already supports tab-completion for ALTER TABLE ADD with constraints
like CHECK and UNIQUE. However, after commit 14e87ffa5c5 introduced
support for adding NOT NULL constraints using ALTER TABLE ADD,
tab-completion for that case was missing.

The attached patch adds tab-completion support for ALTER TABLE ADD NOT NULL.
Thought?

Regards,

-- 
Fujii Masao
NTT DATA Japan Corporation