Fix SCRAM authentication via SSL when mixing versions of OpenSSL

When using a libpq client linked with OpenSSL 1.0.1 or older to connect
to a backend linked with OpenSSL 1.0.2 or newer, the server would send
SCRAM-SHA-256-PLUS and SCRAM-SHA-256 as valid mechanisms for the SASL
exchange, and the client would choose SCRAM-SHA-256-PLUS even if it does
not support channel binding, leading to a confusing error.  In this
case, what the client ought to do is switch to SCRAM-SHA-256 so as the
authentication can move on and succeed.

So for a SCRAM authentication over SSL, here are all the cases present
and how we deal with them using libpq:
1) Server supports channel binding, it sends SCRAM-SHA-256-PLUS and
SCRAM-SHA-256 as allowed mechanisms.
1-1) Client supports channel binding, chooses SCRAM-SHA-256-PLUS.
1-2) Client does not support channel binding, chooses SCRAM-SHA-256.
2) Server does not support channel binding, sends SCRAM-SHA-256 as
allowed mechanism.
2-1) Client supports channel binding, still it has no choice but to
choose SCRAM-SHA-256.
2-2) Client does not support channel binding, it chooses SCRAM-SHA-256.
In all these scenarios the connection should succeed, and the one which
was handled incorrectly prior this commit is 1-2), causing the
connection attempt to fail because client chose SCRAM-SHA-256-PLUS over
SCRAM-SHA-256.

Reported-by: Hugh Ranalli
Diagnosed-by: Peter Eisentraut
Author: Michael Paquier
Reviewed-by: Peter Eisentraut
Discussion: https://postgr.es/m/CAAhbUMO89SqUk-5mMY+OapgWf-twF2NA5sCucbHEzMfGbvcepA@mail.gmail.com
Backpatch-through: 11

src/interfaces/libpq/fe-auth.c

@@ -526,14 +526,24 @@ pg_SASL_init(PGconn *conn, int payloadlen)
 
 		/*
 		 * Select the mechanism to use.  Pick SCRAM-SHA-256-PLUS over anything
-		 * else if a channel binding type is set.  Pick SCRAM-SHA-256 if
-		 * nothing else has already been picked.  If we add more mechanisms, a
-		 * more refined priority mechanism might become necessary.
+		 * else if a channel binding type is set and if the client supports
+		 * it. Pick SCRAM-SHA-256 if nothing else has already been picked.  If
+		 * we add more mechanisms, a more refined priority mechanism might
+		 * become necessary.
 		 */
 		if (strcmp(mechanism_buf.data, SCRAM_SHA_256_PLUS_NAME) == 0)
 		{
 			if (conn->ssl_in_use)
+			{
+				/*
+				 * The server has offered SCRAM-SHA-256-PLUS, which is only
+				 * supported by the client if a hash of the peer certificate
+				 * can be created.
+				 */
+#ifdef HAVE_PGTLS_GET_PEER_CERTIFICATE_HASH
 				selected_mechanism = SCRAM_SHA_256_PLUS_NAME;
+#endif
+			}
 			else
 			{
 				/*