Add libpq connection parameter "scram_channel_binding"

Add libpq connection parameter "scram_channel_binding"

This parameter can be used to enforce the channel binding type used
during a SCRAM authentication.  This can be useful to check code paths
where an invalid channel binding type is used by a client and will be
even more useful to allow testing other channel binding types when they
are added.

The default value is tls-unique, which is what RFC 5802 specifies.
Clients can optionally specify an empty value, which has as effect to
not use channel binding and use SCRAM-SHA-256 as chosen SASL mechanism.

More tests for SCRAM and channel binding are added to the SSL test
suite.

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