s4:lib/tls: add tstream_tls_channel_bindings()

s4:lib/tls: add tstream_tls_channel_bindings()

This is based on GNUTLS_CB_TLS_SERVER_END_POINT
and is the value that is required for channel bindings
in LDAP of active directory domain controllers.

For gnutls versions before 3.7.2 we basically
copied the code from the GNUTLS_CB_TLS_SERVER_END_POINT
implementation as it only uses public gnutls functions
and it was easy to re-implement.

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>