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

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.