BUG/MINOR: stats: Be more strict on what is a valid request to the stats applet

BUG/MINOR: stats: Be more strict on what is a valid request to the stats applet

First of all, only GET, HEAD and POST methods are now allowed. Others will be
rejected with the status code STAT_STATUS_IVAL (invalid request). Then, for the
legacy HTTP, only POST requests with a content-length are allowed. Now, chunked
encoded requests are also considered as invalid because the chunk formatting
will interfere with the parsing of POST parameters. In HTX, It is not a problem
because data are unchunked.

This patch must be backported to 1.9. For prior versions too, but HTX part must
be removed. The patch introducing the status code STAT_STATUS_IVAL must also be
backported.