Move file read/write pointer (INT 21h, 42h - LSEEK)

        Entry:
           AH = 42h
           AL = method value
                  0 = offset from beginning of file
                  1 = offset from present location
                  2 = offset from end of file
           BX = file handle
           CX:DX = offset in bytes
        Return:
          CF = 1 if error
                  AX = Error Code
          CF = 0 successful
                  DX:AX = new offset