MEDIUM: http: implement http-response redirect rules

MEDIUM: http: implement http-response redirect rules

Sometimes it's problematic not to have "http-response redirect" rules,
for example to perform a browser-based redirect based on certain server
conditions (eg: match of a header).

This patch adds "http-response redirect location <fmt>" which gives
enough flexibility for most imaginable operations. The connection to
the server is closed when this is performed so that we don't risk to
forward any pending data from the server.

Any pending response data are trimmed so that we don't risk to
forward anything pending to the client. It's harmless to also do that
for requests so we don't need to consider the direction.