INT 2F - Kingswood TSR Windows - ADD OR REMOVE USER
        AX = 3901h
        BL = 0Bh
        CL = number of users increment: +1 if adding a new user
                                        -1 if removing a user
Return: AX = error code (0000h if successful)
Note:   the TSR windows resident module may only be removed when the internal
          user count is zero
SeeAlso: AX=3901h/BL=02h