BUG/MEDIUM: stats: fix a regression when dealing with POST requests

In 1.5-dev17 (commit 1facd6d6), we reorganized the way HTTP stats
requests are handled. When moving the code, we dropped a "return 0"
which happens upon incomplete POST request, so we now end up with
the next return 1 which causes processing to go on with next
analyser. This causes incomplete POST requests to try to forward
the request to servers, resulting in either a 404 or a 503 depending
on the configuration.

This patch fixes this regression to restore the previous behaviour.
It's not enough though, as it happens that the stats code is handled
after all http header processing but in the same function. The net
effect is that incomplete requests cause the headers manipulation to
be performed multiple times, possibly resulting in multiple headers
in the request buffer. Since the stats requests are not meant to be
forwarded, it's not an issue yet but this is something to take care
of later.

A remaining issue that's not handled yet is that if the client does
not send the complete POST headers, then the request is finally
forwarded. This is not a regression, it has always been there and
seems to be caused by the lack of timeout processing when waiting
for the POST body. The solution to this issue would be to move the
handling of stats requests into a dedicated analyser placed after
http_process_request_body().

Bug reported by Guillaume de Lafond.