Use relative reference URIs in Location header on directory redirects.

Use relative reference URIs in Location header on directory redirects.

This adds support for front-ending httpd(8) with a TLS-terminating
gateway like relayd(8) that forwards unencrypted http traffic.

Previously httpd(8) would use a full URL in the Location header in 301
redirects when a user-agent requests a directory but without the
trailing '/'. If the user-agent originally connected with https, this
caused the redirected url to be http.

This change conforms to RFC7231 section 7.1.2.

Reported by Vincent Lee.

OK claudio@