[MINOR] report haproxy's version by default on the stats page

[MINOR] report haproxy's version by default on the stats page

For people who manage many haproxies, it is sometimes convenient
to be informed of their version. This patch adds this, with the
option to disable this report by specifying "stats hide-version".

Also, the feature may be permanently disabled by setting the
STATS_VERSION_STRING to "" (empty string), or the format can
simply be adjusted.