Add ssl_library preset parameter

This allows querying the SSL implementation used on the server side.
It's analogous to using PQsslAttribute(conn, "library") in libpq.

Reviewed-by: Daniel Gustafsson <daniel@yesql.se>

src/backend/utils/misc/guc.c

@@ -3723,6 +3723,21 @@ static struct config_string ConfigureNamesString[] =
 		check_canonical_path, NULL, NULL
 	},
 
+	{
+		{"ssl_library", PGC_INTERNAL, PRESET_OPTIONS,
+			gettext_noop("Name of the SSL library."),
+			NULL,
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+		},
+		&ssl_library,
+#ifdef USE_SSL
+		"OpenSSL",
+#else
+		"",
+#endif
+		NULL, NULL, NULL
+	},
+
 	{
 		{"ssl_cert_file", PGC_SIGHUP, CONN_AUTH_SSL,
 			gettext_noop("Location of the SSL server certificate file."),