Support TLS handshake directly without SSLRequest negotiation

By skipping SSLRequest, you can eliminate one round-trip when
establishing a TLS connection. It is also more friendly to generic TLS
proxies that don't understand the PostgreSQL protocol.

This is disabled by default in libpq, because the direct TLS handshake
will fail with old server versions. It can be enabled with the
sslnegotation=direct option. It will still fall back to the negotiated
TLS handshake if the server rejects the direct attempt, either because
it is an older version or the server doesn't support TLS at all, but
the fallback can be disabled with the sslnegotiation=requiredirect
option.

Author: Greg Stark, Heikki Linnakangas
Reviewed-by: Matthias van de Meent, Jacob Champion

src/interfaces/libpq/libpq-int.h

@@ -235,7 +235,8 @@ typedef enum
 #define ENC_ERROR			0
 #define ENC_PLAINTEXT		0x01
 #define ENC_GSSAPI			0x02
-#define ENC_NEGOTIATED_SSL	0x04
+#define ENC_DIRECT_SSL		0x04
+#define ENC_NEGOTIATED_SSL	0x08
 
 /* Target server type (decoded value of target_session_attrs) */
 typedef enum
@@ -394,6 +395,8 @@ struct pg_conn
 	char	   *keepalives_count;	/* maximum number of TCP keepalive
 									 * retransmits */
 	char	   *sslmode;		/* SSL mode (require,prefer,allow,disable) */
+	char	   *sslnegotiation; /* SSL initiation style
+								 * (postgres,direct,requiredirect) */
 	char	   *sslcompression; /* SSL compression (0 or 1) */
 	char	   *sslkey;			/* client key filename */
 	char	   *sslcert;		/* client certificate filename */
@@ -563,6 +566,7 @@ struct pg_conn
 
 	/* SSL structures */
 	bool		ssl_in_use;
+	bool		ssl_handshake_started;
 	bool		ssl_cert_requested; /* Did the server ask us for a cert? */
 	bool		ssl_cert_sent;	/* Did we send one in reply? */