prevent sharenfs parameter from breaking NFS exports

knfsd / exportfs will attempt to load any exports file added
to /etc/exports.d. This can lead to filesystems being
unexpectedly exported and a broken NFS configuration.

In this PR, we add an additional check when generating
exports where we remove any unexpected file from
/etc/exports.d and then make the directory immutable.

If for some reason, a file cannot be removed, then we
intentionally generate an invalid exports config with
a comment stating that the user needs to clean up whatever
he or she did there.