Fix compile errors, code review feedback, and add basic libc test suite.

*** 70,82 ****
                  errno = EINVAL;
                  return (NULL);
          }
  
          if (locname == NULL)
!                 return (current_locale(__global_locale, category));
  
!         if ((oldloc = duplocale(__global_locale)) == NULL)
                  return (NULL);
  
          mask = (category == LC_ALL ? LC_ALL_MASK : (1 << category));
  
          newloc = newlocale(mask, locname, oldloc);
--- 70,82 ----
                  errno = EINVAL;
                  return (NULL);
          }
  
          if (locname == NULL)
!                 return (current_locale(___global_locale, category));
  
!         if ((oldloc = duplocale(___global_locale)) == NULL)
                  return (NULL);
  
          mask = (category == LC_ALL ? LC_ALL_MASK : (1 << category));
  
          newloc = newlocale(mask, locname, oldloc);
*** 83,94 ****
          if (newloc == NULL) {
                  freelocale(oldloc);
                  return (NULL);
          }
  
!         oldloc = __global_locale;
!         __global_locale = newloc;
  
          install_legacy(newloc, mask);
  
          freelocale(oldloc);
          return (current_locale(newloc, category));
--- 83,94 ----
          if (newloc == NULL) {
                  freelocale(oldloc);
                  return (NULL);
          }
  
!         oldloc = ___global_locale;
!         ___global_locale = newloc;
  
          install_legacy(newloc, mask);
  
          freelocale(oldloc);
          return (current_locale(newloc, category));