This makes conn.logs a bit prettier (and smaller) because all lines that
do not use a tunnel will now have a "-" instead of the "(empty)" for
tunnel_parents.
This currently supports automatic decapsulation of GTP-U packets on
UDP port 2152.
The GTPv1 headers for such tunnels can be inspected by handling the
"gtpv1_g_pdu_packet" event, which has a parameter of type "gtpv1_hdr".
Analyzer and test cases are derived from submissions by Carsten Langer.
Addresses #690.