selinux: selinuxfs can be mounted on /sys/fs/selinux

selinux: selinuxfs can be mounted on /sys/fs/selinux

The kernel now provides the /sys/fs/selinux mountpoint and libselinux
prefers it if it's available.

systemd currently tests only for /selinux and this leads to an infinite
loop of policy reloads in the latest Rawhide.

Fix it by checking both possible mountpoints.
Also add the new path to ignore_paths[].

/selinux appears also in nspawn.c. I don't think it's necessary to
change it there at this point.

https://bugzilla.redhat.com/show_bug.cgi?id=711015