Refactor channel binding code to fetch cbind_data only when necessary

Refactor channel binding code to fetch cbind_data only when necessary

As things stand now, channel binding data is fetched from OpenSSL and
saved into the SCRAM exchange context for any SSL connection attempted
for a SCRAM authentication, resulting in data fetched but not used if no
channel binding is used or if a different channel binding type is used
than what the data is here for.

Refactor the code in such a way that binding data is fetched from the
SSL stack only when a specific channel binding is used for both the
frontend and the backend.  In order to achieve that, save the libpq
connection context directly in the SCRAM exchange state, and add a
dependency to SSL in the low-level SCRAM routines.

This makes the interface in charge of initializing the SCRAM context
cleaner as all its data comes from either PGconn* (for frontend) or
Port* (for the backend).

Author: Michael Paquier <michael.paquier@gmail.com>