disable Travis CI job using libpcap-dev

The recent PPPoE session ID test case in commit 1ed68a5 depends on the
latest libpcap version to pass. The tcpdump build will fail until the
libpcap feature makes it into the next libpcap release and eventually
into the build server's libpcap-dev package, which won't happen soon.

For the time being leave only the job using git master of libpcap to
build tcpdump, such that Travis status on the pull requests is accurate.
add a test case for Babel protocol with Pad1 TLVs

The packet capture consists of two datagrams produced with a modified
version of Quagga-RE babeld, such that each TLV of the datagrams is
followed with one Pad1 TLV. This encoding is not what a Babel speaker
would typically send, but it is otherwise valid and up to the spec
(RFC6126 Sections 4.2 onwards).

The current version of the decoder has a bug iterating over Pad1.
fix iteration over Babel Pad1 TLVs

The Pad1 TLV consists only of the Type field (RFC6126 Section 4.4.1),
thus the check for Pad1 type must be done before the Length field is
consumed, if ever. This change brings tcpdump decoder into line with the
spec and Babel implementation's own TLV iterator.
initial support for Babel sub-TLVs decoding

Babel sub-TLVs are a work in progress, a backward-compatible extension
to Babel version 2 protocol encoding. This update implements encoding
explained by Juliusz Chroboczek to decode ChanInfo sub-TLV of the Update
TLV.
add minimal OpenFlow decoding framework

This change registers OpenFlow TCP port number and adds processing of
respective packets with openflow_print(), a new function that
understands the minimal OpenFlow header format and can iterate over
messages within a snapshot that starts with the header.
add OpenFlow 1.0 decoder (no SSL)

The new file openflow-1.0.h is a verbatim copy of the file openflow.h
from the openflow-1.0.0.tar.gz distribution. The new file
print-openflow-1.0.c contains a set of functions for OpenFlow 1.0 (wire
protocol 0x01) decoding. Of these functions only
of10_header_body_print() is exported and used by the minimal OpenFlow
decoder.

It is intended that future (1.1, 1.2, 1.3.0) OpenFlow version decoders
are implemented the same way (in modules of their own), since different
versions of OpenFlow specification reuse the same symbols for different
numeric values. This way, print-openflow-1.1.c would include
openflow-1.1.h and so on.

The new test case "of10_p3295-vv" was produced using a Pica8 P-3295
switch and Trema controller running a purpose-built sample application.