Don't use GNodes for the protocol tree, put the sibling pointer, and

Don't use GNodes for the protocol tree, put the sibling pointer, and
pointers to the first *and* last child, in the "proto_node" structure
itself.  That saves us one level of indirection and memory allocation,
and lets us append to a tree by appending to the last child directly,
rather than having to scan through the list of siblings of the first
child to find the end of that list.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@9171 f5534014-38df-0310-8fa8-9805f1628bb7