Interpret X-Forwarded-Port

Under some conditions a reverse proxy may define X-Forwarded-Port yet the Jenkins servlet container still returns 80 for ServletRequest.getServerPort. This causes problems for computing Jenkins.getRootUrlFromRequest, the current implementation of which treats the default port as 80 regardless of the scheme; we would like the default to be 443 when X-Forwarded-Proto: https, yet this will only work if the servlet container coöperates. Easier to have Jenkins itself interpret this header when set.