Add conflicts and provides declarations

This eases the upgrade from selinux- and selinux-usr- packages to new package names
shadow-selinux: move source URL to ftp.al.org

Apply https://projects.archlinux.org/svntogit/packages.git/commit/trunk/PKGBUILD?h=packages/shadow&id=d475ab73a3a662887938d48cae15b0ee94238c92