para_play: Compute the current time more accurately.

Currently get_play_time() throws away the subsecond part of the
timeval and returns a number in seconds. We can improve on that
by letting the function return milliseconds instead. However, with
milliseconds we must perform multiplications using 64 bit integers
to avoid integer overflows.

This also affects the pause and play commands, which should now
reposition the stream more accurately. It still won't be perfect,
though, because play.c has no way of knowing the number of the chunk
which is currently being decoded.