MEDIUM: http: add http-request 'add-header' and 'set-header' to build headers

MEDIUM: http: add http-request 'add-header' and 'set-header' to build headers

These two new statements allow to pass information extracted from the request
to the server. It's particularly useful for passing SSL information to the
server, but may be used for various other purposes such as combining headers
together to emulate internal variables.