Add connection parameters to control SSL protocol min/max in libpq

These two new parameters, named sslminprotocolversion and
sslmaxprotocolversion, allow to respectively control the minimum and the
maximum version of the SSL protocol used for the SSL connection attempt.
The default setting is to allow any version for both the minimum and the
maximum bounds, causing libpq to rely on the bounds set by the backend
when negotiating the protocol to use for an SSL connection.  The bounds
are checked when the values are set at the earliest stage possible as
this makes the checks independent of any SSL implementation.

Author: Daniel Gustafsson
Reviewed-by: Michael Paquier, Cary Huang
Discussion: https://postgr.es/m/4F246AE3-A7AE-471E-BD3D-C799D3748E03@yesql.se

src/interfaces/libpq/libpq-int.h

@@ -367,6 +367,8 @@ struct pg_conn
 	char	   *krbsrvname;		/* Kerberos service name */
 	char	   *gsslib;			/* What GSS library to use ("gssapi" or
 								 * "sspi") */
+	char	   *sslminprotocolversion;	/* minimum TLS protocol version */
+	char	   *sslmaxprotocolversion;	/* maximum TLS protocol version */
 
 	/* Type of connection to make.  Possible values: any, read-write. */
 	char	   *target_session_attrs;