s4:libcli/ldap: add tls channel binding support for ldap_bind_sasl()

s4:libcli/ldap: add tls channel binding support for ldap_bind_sasl()

We still allow 'ldap_testing:tls_channel_bindings = no' and
'ldap_testing:channel_bound = no' for testing
the old behavior in order to have expected failures in our tests.

And we have 'ldap_testing:forced_channel_binding = somestring'
in order to force invalid bindings.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15621

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>