Special case localeconv when NUMERIC, MONETARY must be C

Special case localeconv when NUMERIC, MONETARY must be C

When both these categories are C, we don't need to call localeconv(),
which can be expensive because it may mean toggling locales.  Instead
the answers are well defined by the C Standard.  This adds a function
that returns those, and #ifdef's to locale.c to compile to use the
special function instead of the more general one if both categories must
be C.

A future commit will extend the use of this new function to more cases.