MAJOR: threads: Offically enable the threads support in HAProxy

MAJOR: threads: Offically enable the threads support in HAProxy

Now, USE_THREAD option is implicitly enabled when HAProxy is compiled, for
targets linux2628 and freebsd. To enable it for other targets, you can set
"USE_THREAD=1" explicitly on the command line. And to disable it explicitly, you
must set "USE_THREAD=" on the command line.

Now, to be clear. This does not means it is bug free, far from that. But it
seems stable enough to be tested. You can try to experiment it and to report
bugs of course by setting nbthread parameter. By leaving it to 1 (or not using
it at all), it should be as safe as an HAProxy compiled without threads.

Between the commit "MINOR: threads: Prepare makefile to link with pthread" and
this one, the feature was in development and really unstable. It could be hard
to track a bug using a bisect for all these commits.