The pthread() support was not working. Once I fixed use-thread in a previous push I realized that the pthread/windows code was not working. I've replaced the fork/thread design with a pure pthread design using condition timers and broadcast.

Ramification, UNIX now uses thread, support for slaves had to be dropped and there is no need for the --use-threads flag.
Added --concurrency=0 option so that it will start at 1 and keep going up until something bad happens :)