Allow DUMB to be compiled with C++11, use correct type to some declarations from randi's customized code

Fixes:

```
/home/edward-san/zdoom/trunk/dumb/src/it/readmod.c:656:71: warning: 
      incompatible pointer types passing 'long *' to parameter of type 'int32 *'
      (aka 'int *') [-Wincompatible-pointer-types]
  ...sigdata->n_samples, sigdata->sample, &total_sample_size, &remain);
                                          ^~~~~~~~~~~~~~~~~~
/home/edward-san/zdoom/trunk/dumb/src/it/readmod.c:396:94: note: passing
      argument to parameter 'total_sample_size' here
  ...*f, int n_samples, IT_SAMPLE *sample, int32 *total_sample_size, int32 *r...
                                                  ^
/home/edward-san/zdoom/trunk/dumb/src/it/readmod.c:656:91: warning: 
      incompatible pointer types passing 'long *' to parameter of type 'int32 *'
      (aka 'int *') [-Wincompatible-pointer-types]
  ...sigdata->n_samples, sigdata->sample, &total_sample_size, &remain);
                                                              ^~~~~~~
/home/edward-san/zdoom/trunk/dumb/src/it/readmod.c:396:120: note: passing
      argument to parameter 'remain' here
  ...n_samples, IT_SAMPLE *sample, int32 *total_sample_size, int32 *remain)
                                                                    ^
/home/edward-san/zdoom/trunk/dumb/src/it/readxm.c:474:2: warning: incompatible
      pointer types initializing 'int32 (*)(char *, int32, void *)' with an
      expression of type 'long (*)(char *, int32, void *)'
      [-Wincompatible-pointer-types]
        &limit_xm_getnc,
        ^~~~~~~~~~~~~~~
```