* fhandler.h (class dev_console): Constify charset parameter of

	* 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.