INT 21 - European MSDOS 4.0 - "FREEZE" - STOP A PROCESS
        AH = 81h
        BX = flag (00h freeze command subtree, 01h only specified process)
        CX = Process ID of head of command subtree
Return: CF clear if successful
        CF set on error
            AX = error code (no such process)
Desc:   temporarily suspend a process or a process and all of its children
Note:   if BX=0001h, this call will not return until the process is actually
          frozen, which may not be until after it unblocks from an I/O
          operation
SeeAlso: AH=89h,AX=8E00h,INT 15/AX=101Dh