INT 21 - LANtastic v4.0+ - STUFF SERVER KEYBOARD BUFFER
        AX = 5FCAh
        ES:DI -> ASCIZ server name in form "\\machine"
        DS:SI -> ASCIZ string to stuff (128 bytes)
Return: CF clear if successful
        CF set on error
            AX = error code
Note:   you must have the "S" privilege to use this call
        maximum number of characters that can be stuffed is determined by the
          server's RUN BUFFER SIZE.