T: increase max number of args in an event and make the define more private

T_MAX_ARGS was 16 and some people start to use the T machinery with
more than 16 values. So be it. Let tracers be friendly and accept 32.
(More commits to follow for full support.)

Also why is T_MAX_ARGS in T_defs.h when it's only used by event.h?
Hum?

common/utils/T/tracer/event.h

@@ -7,6 +7,9 @@
 #include <time.h>
 #endif
 
+/* maximum number of arguments for the T macro */
+#define T_MAX_ARGS 32
+
 enum event_arg_type {
   EVENT_INT,
   EVENT_ULONG,