Have different MTP2 dissectors for different metadata and CRC.

Have separate MTP2 dissectors for:

	MTP2 frames with no pseudo-header and no CRC;

	MTP2 frames with a pseudo-header and no CRC;

	MTP2 frames with no pseudo-header and a CRC;

and call the appropriate dissector in the appropriate places.

While we're at it, get rid of a global variable - pass the "use extended
sequence numbers" flag down through the dissection code path, rather
than having it as a global.

Change-Id: Id8da1fbe3529e3ffadd5c30646cbc922f506a01f
Reviewed-on: https://code.wireshark.org/review/5679
Reviewed-by: Guy Harris <guy@alum.mit.edu>