Add more information to the WTP protocol summary (PDU type and PDU specific

Add more information to the WTP protocol summary (PDU type and PDU specific
important parameters).

Document the computation of the length field in WTP concatenation after having
looked at a capture with the length field encoded as a WSP uintvar-integer.

Use "common code" for WTP reassembly, by calling process_reassembled_data().
Document the behavior of reassembly as the output of Ethereal differs between
the first and the second pass.

Question: shoud the common reassembly code provide a call-back mechanism to
get access to previously-unreassembled packets that appear to be part of a
reassembled whole, and to be able to update the state and information of
those packets at the time of the reassembly?

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@9547 f5534014-38df-0310-8fa8-9805f1628bb7