Add selinux-* conflicts and provides declarations in PKGBUILD files

Hello,

To make the upgrade from an installation with ``selinux-*`` packages to the new packages (``lib*`` and ``*-selinux``), I've added some ``conflicts`` and ``provides`` variables to every package. Could you merge this to the AUR?

Moreover my system refused to install ``systemd-selinux`` as this package broke dependencies because its ``provides`` variable was overwritten. The second patch fixes this issue.

Thanks,

Nicolas