Improve error checking and error messages in string-collation functions.
src/sysdep.c (str_collate) [__STDC_ISO_10646__]: Improve the
wording of the error messages.
(str_collate) [WINDOWSNT]: Signal an error if w32_compare_strings
sets errno.
src/w32proc.c (get_lcid_callback): Accept locale specifications
without the country part, as in "enu" vs "enu_USA".
(w32_compare_strings): Signal an error if a locale was specified,
but couldn't be translated into a valid LCID.