treat the "big5hkscs" encoding as an East Asian encoding, which uses

* src/iso2022.c: treat the "big5hkscs" encoding as an East Asian encoding,
	which uses ambiguous widths (#116078).  Report and patch from Roger So.
* src/dumpkeys.c, src/iso2022.c, src/iso8859mode.c, src/nativeecho.c,
	src/utf8echo.c, src/utf8mode.c, src/vte.c: don't use the literal escape
	character, express it as an octal sequence.