use proto_tree_add_item() for IPv6 header, to allow filtering like below

use proto_tree_add_item() for IPv6 header, to allow filtering like below
possible.
	ipv6.nxt == 17
	ipv6.dst == ff02::9
	ipv6.src[0:2] == fe:80

modify dfilter lexical rule to allow standard IPv6 expression to be
passed up to parser.

XXX backward compat issue in lex rule, maybe
XXX IPv6 has chained headers.  how will dfilter behave when we have
multiple protocol header of the same type?
XXX ipv6.nxt is not really useful due to IPv6 chained header.  we need a
symbol to identify "final" protocol type on the chain (testing ipv6.final
but will SEGV).

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@836 f5534014-38df-0310-8fa8-9805f1628bb7