cdt: use 'size_t' types for counting stats instead of 'int'

cdt: use 'size_t' types for counting stats instead of 'int'

This squashes two compiler warnings:

  dtstat.c: In function ‘dtstat’:
  dtstat.c:60:59: warning: conversion to ‘long unsigned int’ from ‘int’ may
    change the sign of the result [-Wsign-conversion]
     60 |  if(!(Count = malloc((ds->dt_max+1)*sizeof(int))) )
        |                                    ^
  dtstat.c:74:65: warning: conversion to ‘long unsigned int’ from ‘int’ may
    change the sign of the result [-Wsign-conversion]
     74 |  if(!(Count = malloc((ds->dt_n+1)*sizeof(int))) )
        |                                  ^

It also allows these functions to now count beyond 2³¹ - 1.