truncating string copy

truncating string copy

     The majority of our calls to strncpy are in the form
  (void) strncpy(dst, src, n);
  dst[n] = '\0';
so add a new routine, copynchars, which does that.  A few calls care
about strncpy's return value and at least one relies on it only copying a
substring without also terminating the output, but most don't care about
either and none seem to care that `n' ought to have type size_t instead of
int.  The new routine matches our usage better, but I haven't gone through
to change the existing strncpy calls.