Wipe out the container's /sys/fs/selinux to not advertise SELinux

This is the second time a Podman regression has caused a selinuxfs
instance to leak into the toolbox container's /sys/fs/selinux,
tricking various components into trying to use SELinux. It might be
better to work this around in Toolbox until the situation in Podman is
figured out.

Based on an idea from Colin Walters.

https://github.com/containers/libpod/issues/4452

toolbox

@@ -1201,6 +1201,10 @@ init_container()
                 return 1
             fi
 
+            if ! mount_bind /usr/share/empty /sys/fs/selinux; then
+                return 1
+            fi
+
             if ! mount_bind /run/host/var/lib/flatpak /var/lib/flatpak ro; then
                 return 1
             fi