#18 one-way Tmix addition

[Michele Weigle]

This patch adds support for using one-way TcpAgents with the Tmix traffic generator, which previously only worked with FullTcp agents.
This patch also includes the relevant updates to documentation (doc/tmix.tex), examples (tcl/ex/tmix/text-tmix-oneway.tcl), and tests
(tcl/test/test-suite-tmix.tcl; tcl/test/test-output/tmix/).

The new commands for Tmix objects are "set-sink" and "set-agent-type". See doc/tmix.tex for details.

There is one minor change to non-Tmix code: TcpAgent::reset() was made public in tcp/tcp.h. This is so both full-tcp and one-way agents could
be reused by Tmix during the simulation.

[Michele Weigle]

one-way Tmix