Revert "Remove support of RPAFsetport"

This reverts commit eab06b2
I will add a note there are some limitation for this option.

> The feature does work, but you can't mix and match ports. So if you
have one virtualhost on localhost:80, and you only send requests like
X-Forwarded-Port: 443, then it works. But, if you send a mixture of
regular and X-Forwarded-Port: 80 it won't work.

Thank you @chmac !
Add note for RPAFsetport limitation