cygheap.h:
- Replace 'charset' field in cygheap_locale with 'int charset_index'

cygheap.cc:
- Initialise 'charset_index' to zero

winsup/cygwin/wchar.h:
- Change mbtowc_f and wctomb_f typedefs to take an int argument instead of the
  charset string
- Same for the sys_cp_wcstombs/mbstowcs prototypes
- add extern declaration for __charset_index

fhandler.h:
- Replace charset argument to str_to_con with 'int cs_idx'

fhandler_console.cc:
- Reflect changes above in function definitions and calls
- In write_normal(), replace 'charset' local with 'int cs_idx'. Where currently
  'charset' is set to "CP437", set cs_idx to 0 (which is the index for CP437)

winsup/cygwin/strfuncs.cc
- Replace the charset arguments of all the __xxx_wctomb/mbtowc functions
- Same for sys_cp_wcstombs/mbstowcs
- When testing whether the selected charset is UTF-8, do so by comparing
  f_mbtowc/wctomb against __utf8_mbtowc/wctomb
- Pass 0 as the charset index in direct calls to __utf8_mbtowc/wctomb

winsup/cygwin/nlsfuncs.cc:
- Reflect header changes above
- Also change these functions to take a charset index instead of a string:
  lc_wcstombs, lc_mbstowcs, __getlocaleinfo, __eval_datetimefmt,
  __set_lc_time_from_win, __set_lc_monetary_from_win, __collate_load_locale
- Change calls to affected functions. There's a lot of them
- In __set_locale_from_locale_alias, use index '-1' instead of "ISO-8859-1"
  in call to lc_mbstowcs
- In internal_setlocale, instead of __locale_charset(), compare __mbtowc and
  __charset_index with the corresponding cygheap values to see whether the
  charset has changed. Similar for ASCII test a bit further down
- Copy __charset_index rather than __charset string to cygheap