INT 2F U - DOS 3.3+ internal - MOVE FILE POINTER
        AX = 1228h
        BP = 4200h, 4201h, 4202h (see INT 21/AH=42h)
        BX = file handle
        CX:DX = offset in bytes
        SS = DOS DS (must be using a DOS internal stack)
Return: as for INT 21/AH=42h
Notes:  equivalent to INT 21/AH=42h, but may only be called from inside a DOS
          function call
        sets user stack frame pointer to dummy buffer, moves BP to AX, performs
          LSEEK, and restores frame pointer