s4:libcli/ldap: just use gensec_update() in ldap_bind_sasl()

s4:libcli/ldap: just use gensec_update() in ldap_bind_sasl()

We're in a blocking/sync call, we should avoid using nested event loops for
this. As far as I can see ldap_bind_sasl() is only called from command line
tools, which are ok to block.

Resolving this requires also resolving the general case in LDB, as that is the
API this is used from.  We would need ldb_connect_send() and ldb_connect_recv()
at a start.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Mon May 22 01:12:23 CEST 2017 on sn-devel-144