Rename deref_aliases back to ldap_deref_aliases

The "ldap_deref_aliases" option has accidentally been renamed to
"deref_aliases".  Revert that change (but accept both names for a
while), so that the option name now matches the documentation again.
Merge pull request #302 from weiss/ldap-deref-aliases

Rename deref_aliases back to ldap_deref_aliases