MEDIUM: http-rules: Support extra headers for HTTP return actions

MEDIUM: http-rules: Support extra headers for HTTP return actions

It is now possible to append extra headers to the generated responses by HTTP
return actions, while it is not based on an errorfile. For return actions based
on errorfiles, these extra headers are ignored. To define an extra header, a
"hdr" argument must be used with a name and a value. The value is a log-format
string. For instance:

  http-request status 200 hdr "x-src" "%[src]" hdr "x-dst" "%[dst]"