INT 2F - MDEBUG command driver - EXECUTE KEY IN THE INTERPRETER                 
	AX = D104h
	DS:SI -> MDEBUG data structure (see #03078)
Return: AL = FFh
	AH = return code
	    00h key processed, read next key
	    01h leave MDEBUG popup session
	    02h leave MDEBUG popup session and automactically popup again if
		  DOS is not busy
	    03h signal an error (beep)
	    04h driver has redefined the key, proceed with the new key
		BX = new key code
		MDEBUG won't pass the new key to the command driver(s)
	    05h put new command line into the input buffer
		DS:SI -> new command line (ASCIZA NUL-terminated ASCII string.	The ASCIZ string "ABC" consists of the four bytes 41h, 42h, 43h, and 00h.  Unless otherwise specified, maximum lengths given in the interrupt list do not include the terminating NUL. string, max 66 chars)
	    06h process new command line
		DS:SI -> new command line (ASCIZA NUL-terminated ASCII string.	The ASCIZ string "ABC" consists of the four bytes 41h, 42h, 43h, and 00h.  Unless otherwise specified, maximum lengths given in the interrupt list do not include the terminating NUL. string, max 66 chars)
	   else treat like code 00h
Note:	this function must end with a far call to the old INT 2F handler if the
	  driver does not support the key in BX.  Otherwise, the driver must
	  not chain to the old INT 2F.
SeeAlso: AX=D103h