Update external mode flags (EX_*), changing EX_INR to EX_STDIN, EX_OUTR to

EX_STDOUT, defining EX_STDIO (to be both STDIN and STDOUT), adding XTRN_CONIO
and EX_CONIO (for future cioxtrn/doorway mode on Windows), change IO_INTS to
XTRN_STDIO