INT 2F U - DOS 4.x only SHELLB.COM - SHELLC.EXE INTERFACE
        AX = 1901h
        BL = 00h if SHELLC transient
             01h if SHELLC resident
        DS:DX -> far call entry point for resident SHELLC.EXE
Return: ES:DI -> SHELLC.EXE workspace within SHELLB.COM
Note:   SHELLB.COM and SHELLC.EXE are parts of the DOS 4.x shell