trace: fix string conversion error on python 2.7.5

On python 2.7.5 ptyhon complains about struct() not accepting unicode
argument. Fix by adding explicit conversion.
trace: disable port number conversion in tcpdump mode
trace: add dpkt module

dpkt is a module which is used by 'trace.py tcpdump' and 'trace.py
list --tcpdump' to convert captured packets to PCAP format.

This change is about making trace.py work out of the box and require
no manual work of installing the dependency.

 Source:  https://code.google.com/p/dpkt/
 License: 3-clause BSD
 Size:    430K
tests: introduce tracing smoke test

The test verifies that:
 * samples are collected by OSv
 * extraction via `trace extract` succeeds
 * `trace summary` shows expected tracepoint names
 * listing via `trace list` shows expected tracepoints
 * network packet parsing works

This test is run as part of standard 'make check'.

Note: this single test can be executed as easily as:

 $ scripts/test.py --test tracing_smoke_test
hack: print os environ