When computing the progress, we perform some arithmetics that are
implicitly converting from `size_t` to `int`. In one case we're
calclulating a percentage, so we know that it should always be in the
range of [0,100] and thus we're fine. In the other case we convert from
bytes to kilobytes -- this should be stored in a `size_t` to avoid loss
of precision, even though it probably won't matter due to limited
download rates.