Add format checks for printf-style functions

Added format checks to functions that expect printf style arguments and
fixed any broken calls to these functions.

include/maxscale/cdefs.h

@@ -79,6 +79,17 @@
  */
 #define MXS_MAX(a,b) ((a)>(b) ? (a) : (b))
 
+/**
+ * Define function attributes
+ *
+ * The function attributes are compiler specific.
+ */
+#ifdef __GNUC__
+#define mxs_attribute(a) __attribute__(a)
+#else
+#define mxs_attribute(a)
+#endif
+
 /**
  * COMMON INCLUDE FILES
  */