Add an additional "protocol index" argument to "{old_}dissector_add()",

Add an additional "protocol index" argument to "{old_}dissector_add()",
"{old_}heur_dissector_add()", "{old_}conv_dissector_add()", and
"register_dissector()", so that an entry in those tables has associated
with it the protocol index of the protocol the dissector handles (or -1,
if there is no protocol index for it).

This is for future use in a number of places.

(Arguably, "proto_register_protocol()" should take a dissector pointer
as an argument, but

	1) it'd have to handle both regular and heuristic dissectors;

	2) making it take either a "dissector_t" or a union of that and
	   a "heur_dissector_t" introduces some painful header-file
	   interdependencies

so I'm punting on that for now.  As with other Ethereal internal APIs,
these APIs are subject to change in the future, at least until Ethereal
1.0 comes out....)

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@2849 f5534014-38df-0310-8fa8-9805f1628bb7