Back-patch libpq support for TLS versions beyond v1.

Since 7.3.2, libpq has been coded in such a way that the only SSL protocol
it would allow was TLS v1.  That approach is looking increasingly obsolete.
In commit 820f08ca we fixed it to allow TLS >= v1, but did not
back-patch the change at the time, partly out of caution and partly because
the question was confused by a contemporary server-side change to reject
the now-obsolete SSL protocol v3.  9.4 has now been out long enough that
it seems safe to assume the change is OK; hence, back-patch into 9.0-9.3.

(I also chose to back-patch some relevant comments added by commit
326e1d73, but did *not* change the server behavior; hence, pre-9.4
servers will continue to allow SSL v3, even though no remotely modern
client will request it.)

Per gripe from Jan Bilek.

src/backend/libpq/be-secure.c

@@ -735,6 +735,13 @@ initialize_SSL(void)
 #endif
 		SSL_library_init();
 		SSL_load_error_strings();
+
+		/*
+		 * We use SSLv23_method() because it can negotiate use of the highest
+		 * mutually supported protocol version, while alternatives like
+		 * TLSv1_2_method() permit only one specific version.  Note that we
+		 * don't actually allow SSL v2, only v3 and TLS protocols (see below).
+		 */
 		SSL_context = SSL_CTX_new(SSLv23_method());
 		if (!SSL_context)
 			ereport(FATAL,

src/interfaces/libpq/fe-secure.c

@@ -967,7 +967,13 @@ init_ssl_system(PGconn *conn)
 			SSL_load_error_strings();
 		}
 
-		SSL_context = SSL_CTX_new(TLSv1_method());
+		/*
+		 * We use SSLv23_method() because it can negotiate use of the highest
+		 * mutually supported protocol version, while alternatives like
+		 * TLSv1_2_method() permit only one specific version.  Note that we
+		 * don't actually allow SSL v2 or v3, only TLS protocols (see below).
+		 */
+		SSL_context = SSL_CTX_new(SSLv23_method());
 		if (!SSL_context)
 		{
 			char	   *err = SSLerrmessage();
@@ -982,6 +988,9 @@ init_ssl_system(PGconn *conn)
 			return -1;
 		}
 
+		/* Disable old protocol versions */
+		SSL_CTX_set_options(SSL_context, SSL_OP_NO_SSLv2 | SSL_OP_NO_SSLv3);
+
 		/*
 		 * Disable OpenSSL's moving-write-buffer sanity check, because it
 		 * causes unnecessary failures in nonblocking send cases.