Communicating Applications Spec - Open file

        Entry:
          AX = CB07h
          BX = event handle
          CX = receive file number (ignored for task queue and log queue)
              0000h  open receive control file
              N      open Nth received data file
          DL = queue
              00h task queue
              01h receive queue control file or received file, as given by CX
              02h log queue
        Return:
          AX = 0000h successful
            BX = DOS file handle for requested file
          < 0    error code (see AX=CB01h)