Have a heur_dissector_list_t be an opaque handle.

This allows dissector lists to be looked up by name, so they can be
shared by multiple dissectors.

(This means that there's no "udplite" heuristic dissector list, but
there shouldn't be one - protocols can run atop UDP or UDPLite equally
well, and they share a port namespace and uint dissector table, so they
should share a heuristic dissector table as well.)

Change-Id: Ifb2d2c294938c06d348a159adea7a57db8d770a7
Reviewed-on: https://code.wireshark.org/review/5936
Reviewed-by: Guy Harris <guy@alum.mit.edu>