MEDIUM: freq-ctr: use explicit-size types for freq-ctr struct

freq-ctr struct is used by the shm_stats_file API, and more precisely,
it is used in the shm_stats_file_object struct for counters.

shm_stats_file_object struct requires to be plateform-independent, thus
we switch to using explicit size types (AKA fixed width integer types)
for freq-ctr, in the attempt to make freq-ctr size and memory mapping
consistent from one platform to another.

We cannot simply use fixed-width integer because some of them are
involved in atomic operations, and forcing a given width could
cause build issues on some platforms where atomic ops are not
implemented for large integers. Instead we leverage the FIXED_SIZE
macro to keep handling the integers as before, but forcing them to
be stored using expected number of bytes (unused bytes will simply
be ignored).

No change of behavior should be expected.

include/haproxy/freq_ctr-t.h

@@ -28,11 +28,16 @@
  * period has to be known by the user. The period is measured in ticks and
  * must be at least 2 ticks long. This form is slightly more CPU intensive for
  * reads than the per-second form as it involves a divide.
+ *
+ * /!\ any change performed here will impact shm-stats-file mapping because the
+ * struct is embedded in shm_stats_file_object struct, so proceed with caution
+ * and change shm stats file version if needed. Also, fixed width integer types
+ * should be used (for portability)
  */
 struct freq_ctr {
-	unsigned int curr_tick; /* start date of current period (wrapping ticks) */
-	unsigned int curr_ctr; /* cumulated value for current period */
-	unsigned int prev_ctr; /* value for last period */
+	FIXED_SIZE(4, unsigned int, curr_tick); /* start date of current period (wrapping ticks) */
+	FIXED_SIZE(4, unsigned int, curr_ctr);  /* cumulated value for current period */
+	FIXED_SIZE(4, unsigned int, prev_ctr);  /* value for last period */
 };
 
 #endif /* _HAPROXY_FREQ_CTR_T_H */