improve ZeroMQ support (ZMTP/1.0 inside PGM/EPGM)

This change adds new code to decode ZeroMQ datagrams, couples it with
the PGM decoder and extends the -T option to make all this work.

There are two new test cases based on existing captures of ZMTP/1.0
inside [E]PGM to decode the ZMTP/1.0 part of these.

This functionality enables decoding of the traffic zeromq library
produces for "pgm://" and "epgm://" protocol schemas.