named locale error checking

[Benjamin Kosnik]

> The locales supplied are part of the "implementation".  If they are
> not complete, that seeems like the implementation's problem; what is 
> the user supposed to do about it?  To me, if the implemention comes 
> with incomplete locales, then the implementation should tolerate 
> incomplete locales gracefully.  That means the implementation should 
> provide reasonable defaults for values not supplied.

This appears to be an area with considerable implementation leeway. 
Therefore, for --enable-clocale=gnu, I'll do the error checking for 
incomplete locales as explained previously and document expected behavior.

For other locale models, more elaborate constructions are possible.

To me, substituting 'default' values into a named locale defeats the 
purpose of named locales entirely. 

-benjamin