Make the ENTTEC dissector reject packets that don't look like ENTTEC packets.

This should avoid questions like

	https://ask.wireshark.org/questions/38198/what-is-enttec-in-a-pcap-file

It also splits DMX-over-UDP and DMX-over-TCP into separate dissectors,
as

	1) DMX-over-TCP has only config packets, DMX-over-UDP has the
	   others;

	2) that would let us do reassembly, if necessary, for
	   DMX-over-TCP.

Change-Id: I2606c814693028c7ba2bbc458e45c853372baaf3
Reviewed-on: https://code.wireshark.org/review/5522
Reviewed-by: Guy Harris <guy@alum.mit.edu>