* tools: add tcptracer

This allows tracking TCP connections by tracking TCP connects, closes
and accepts.

This is different from existing tools like tcpconnect or tcpaccept in
that:

* It includes more information like network namespace or source ports
for tcpconnects or remote ports for tcpaccepts
* It traces tcp_close allowing to see when a connection ends
* It only shows information about established connections

* tcptracer: add to README

* tcptracer: add example

* tcptracer: add man page