Introduce 'relative' scheme to return only the paths for home_url, site,...

Introduce 'relative' scheme to return only the paths for home_url, site, admin, network_, and get_ variants. props SergeyBiryukov, see #18952.

git-svn-id: https://core.svn.wordpress.org/trunk@19870 1a063a9b-81f0-0310-95a4-ce76da25c4cd