Expect mixups between non-ECM and ECM image modes.

The TCF data should always be sent as non-ECM image data, whether the images which follow it are sent as non-ECM or ECM data. However, some broken implementations of T.38 (e.g. Mediatrix) may send an hdlc-sig-end message to terminate the TCF data. To be as tolerant as possible, it is probably wise to accept hdlc-sig-end or t4-non-ecm-sig-end, regardless of which kind of data it is really terminating.