ipaldap: merge external_bind into LDAPClient
* Rename do_external_bind to external_bind
* Remove user_name argument in external_bind() and always set it
to effective user name
https://fedorahosted.org/freeipa/ticket/6461
Reviewed-By: Martin Basti <mbasti@redhat.com>
Reviewed-By: Jan Cholasta <jcholast@redhat.com>