r21135: Instead of having hooks to update keytabs as an explicit thing, update

r21135: Instead of having hooks to update keytabs as an explicit thing, update
them as a hook on ldb modify, via a module.

This should allow the secrets.ldb to be edited by the admin, and to
have things update in the on-disk keytab just as an in-memory keytab
would.

This isn't really a dsdb plugin, but I don't have any other good ideas
about where to put it.

Andrew Bartlett