* fhandler.h (class dev_console): Constify charset parameter of
str_to_con.
* fhandler_console.cc (dev_console::con_to_str): Simplify. Always
default to the current internal locale.
(dev_console::get_console_cp): Always use codepage 437 for alternate
charset.
(dev_console::str_to_con): Constify charset parameter.
(fhandler_console::write_normal): Always use codepage 437 for alternate
charset. Otherwise always default to the current internal locale.
Replace ASCII SO with ASCII CAN.
* strfuncs.cc: Tweka comments according to below changes.
(sys_cp_wcstombs): Constify charset parameter. Convert all wchar_t
values in the Unicode private use area U+F0xx to the singlebyte
counterpart. Drop special handling creating ASCII SO sequence from
U+DCxx value. Rearrange for performance. Replace ASCII SO with
ASCII CAN.
(sys_cp_mbstowcs): Constify charset parameter. Replace ASCII SO with
ASCII CAN. Drop special case for U+DCxx ASCII SO sequences. Always
create a replacement from the Unicode private use area U+F0xx for
invalid byte values in a multibyte sequence. Do the same for wchar_t
values from the U+F0xx range to make them roundtrip safe.
* wchar.h (sys_cp_wcstombs): Constify charset parameter.
(sys_cp_mbstowcs): Ditto.