MINOR: http: Action for manipulating the returned status code.

MINOR: http: Action for manipulating the returned status code.

This patch is inspired by Bowen Ni's proposal and it is based on his first
implementation:

   With Lua integration in HAProxy 1.6, one can change the request method,
   path, uri, header, response header etc except response line.
   I'd like to contribute the following methods to allow modification of the
   response line.

   [...]

   There are two new keywords in 'http-response' that allows you to rewrite
   them in the native HAProxy config. There are also two new APIs in Lua that
   allows you to do the same rewriting in your Lua script.

   Example:
   Use it in HAProxy config:
   *http-response set-code 404*
   Or use it in Lua script:
   *txn.http:res_set_reason("Redirect")*

I dont take the full patch because the manipulation of the "reason" is useless.
standard reason are associated with each returned code, and unknown code can
take generic reason.

So, this patch can set the status code, and the reason is automatically adapted.