C/Strncatf

This is a variation of strcat() that allows you to pass a formatted string like printf() and max length for dest, not for how much of src to use.

char *strncatf(char *dest, int length, char *format, ...) {
  char buf[4096];
  va_list ap;
  va_start(ap,format);
  vsnprintf(buf,sizeof(buf),format,ap);
  va_end(ap);
  length -= strlen(dest);
  strncat(dest,buf,length-1);
}