Rename deref_aliases back to ldap_deref_aliases

The `ldap_deref_aliases` option has [accidentally][1] been renamed to `deref_aliases`.  Revert that change (but accept both names for a while), so that the option name now matches the [documentation][2] again.

Thanks to Adi Kriegisch for noting the issue!

[1]: https://github.com/processone/ejabberd/commit/9deb294328bb3f9eb6bd2c0e7cd500732e9b5830
[2]: http://www.process-one.net/docs/ejabberd/guide_en.html#ldap