INT 16 - Shamrock Software EMAIL - MONITOR XMODEM DOWNLOAD
        AX = 4506h
        DL = port number (01h = COM1)
        ES:BX -> 13-byte buffer for ASCIZ filename
Return: AX = 4D00h if EMAIL installed on specified port
            DH = Xmodem status
                00h no XGET command given
                01h XGET in progress
                02h XGET completed successfully
            ES:BX buffer filled with last filename given to XGET command
                (without path)
Note:   DH=02h will only be returned once per XGET; subsequent calls will
          return DH=00h
SeeAlso: INT 17/AX=2408h