Rename 'invalid' to 'disqualified' in the 'show rib' table.

Rename 'invalid' to 'disqualified' in the 'show rib' table.

'invalid' conflicts with the 'in' keyword and the parser is not smart
enough to handle this. As a secondary benefit the term 'invalid' is
less overloaded. There are various reasons why prefixes are not eligible
in the route decision process calling them all 'invalid' is a bit harsh.

job@, tb@ and benno@ agree