t/loc_tools.pl: Fix some bugs in locales_enabled()

t/loc_tools.pl: Fix some bugs in locales_enabled()

This code assumed that all locale categories were represented by
non-negative whole numbers.  However, it turns out that this assumption
is wrong, as on AIX, LC_ALL is -1.  This commit changes our assumption to
take into account that reality; it now assumes that all categories are
larger than a much more negative number, and now the new assumption is
tested for, and if wrong, the code dies instead of silently doing the
wrong thing.

There was also a bug where if a locale category wasn't defined on the
machine, but the corresponding #ifdef for using that category was still
set, the category was improperly assumed to exist