Add tvbuff class.

Add tvbuff class.
Add exceptions routines.
Convert proto_tree_add_*() routines to require tvbuff_t* argument.
Convert all dissectors to pass NULL argument ("NullTVB" macro == NULL) as
the tvbuff_t* argument to proto_tree_add_*() routines.

dissect_packet() creates a tvbuff_t, wraps the next dissect call in
a TRY block, will print "Short Frame" on the proto_tree if a BoundsError
exception is caught.

The FDDI dissector is converted to use tvbuff's.

svn path=/trunk/; revision=1939