sdk: add generic X return code

This type is should be used instead of plain int for all functions
returning and X error code (that's possibly sent to the client),
in order to make return value semantics clear to the humen reader.
(for the compiler, it still will be just an int)

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>

include/misc.h

@@ -102,6 +102,16 @@ OF THIS SOFTWARE.
 
 typedef uint32_t ATOM;
 
+/* @brief generic X return code
+ *
+ * this type is should be used instead of plain int for all functions
+ * returning and X error code (that's possibly sent to the client),
+ * in order to make return value semantics clear to the humen reader.
+ *
+ * part of public SDK / driver API.
+ */
+typedef int XRetCode;
+
 #ifndef TRUE
 #define TRUE 1
 #define FALSE 0