Fix GCC string truncation warnings

Fix GCC string truncation warnings

- Fixes the following GCC compiler warnings:
  rrd_fetch.c:338:9: warning: 'strncpy' output may be truncated copying
  19 bytes from a string of length 19 [-Wstringop-truncation]
    strncpy((*ds_namv)[i], rrd.ds_def[i].ds_nam, DS_NAM_SIZE - 1);
  rrd_tune.c:284:13: warning: 'strncpy' output may be truncated copying
  19 bytes from a string of length 19 [-Wstringop-truncation]
    strncpy(rrd.ds_def[ds].ds_nam, ds_new, DS_NAM_SIZE - 1);
  rrd_graph.c:2282:17: warning: 'strncpy' output may be truncated
    copying 204 bytes from a string of length 204
    [-Wstringop-truncation] strncpy(im->gdes[i].legend, saved_legend,
    sizeof im->gdes[0].legend - 1);