locale.c: Add, move some comments, and a declaration

locale.c: Add, move some comments, and a declaration

This adds some more comments at the beginning of a function concerning
its API, and moves them to before any declarations.

It also moves the declaration for 'done' to the block of other
declarations, and adds a PERL_UNUSED_VAR call if the code that uses it
is #ifdef'd out.  Previously it was too easy to not notice the
declaration separate from the others, and to insert code between the
two, which would not compile under C89, but only on Ultrix machines.