userspace/tools: added a program to control the TX on a WRS port

- wrs_port_tx_control added.
This program controls the TX laser of an SFP by means of the hardware TX
enable line of the SFP. This line is switched via the FPGA.

Signed-off-by: Tjeerd Pinkert <t.j.pinkert@alumnus.utwente.nl>