INT 21 - DOS 2+ internal - CREATE CHILD PSP
        AH = 55h
        DX = segment at which to create new PSP
        SI = (DOS 3+) value to place in memory size field at DX:[0002h]
Return: CF clear if successful

Notes:  creates a "child" PSP rather than making an exact copy of the current
          PSP; the new PSP's parent pointer is set to the current PSP and the
          reference count for each inherited file is incremented
        (DOS 2+) sets current PSP to DX
        (DOS 3+) marks "no inherit" file handles as closed in child PSP
SeeAlso: AH=50h