Add some simple sanity checking of BVLC packets, so if you're unlucky

Add some simple sanity checking of BVLC packets, so if you're unlucky
enough to have a non-BVLC packet from or to port 47808, we're likely to
reject it and let another dissector try it.  Fixes bug 855 (in which an
MS Messenger packet was unlucky enough to have come from that port).

Add value_string tables to some fields, and just use
proto_tree_add_uint() to add them to the protocol tree, rather than
generating our own text for them.

svn path=/trunk/; revision=18033