MEDIUM: http-rules: Add the return action to HTTP rules

Thanks to this new action, it is now possible to return any responses from
HAProxy, with any status code, based on an errorfile, a file or a string. Unlike
the other internal messages generated by HAProxy, these ones are not interpreted
as errors. And it is not necessary to use a file containing a full HTTP
response, although it is still possible. In addition, using a log-format string
or a log-format file, it is possible to have responses with a dynamic
content. This action can be used on the request path or the response path. The
only constraint is to have a responses smaller than a buffer. And to avoid any
warning the buffer space reserved to the headers rewritting should also be free.

When a response is returned with a file or a string as payload, it only contains
the content-length header and the content-type header, if applicable. Here are
examples:

  http-request return content-type image/x-icon file /var/www/favicon.ico  \
      if { path /favicon.ico }

  http-request return status 403 content-type text/plain    \
      lf-string "Access denied. IP %[src] is blacklisted."  \
      if { src -f /etc/haproxy/blacklist.lst }
3 files changed