Replace podcast.log atomically, create temp log dynamically.

Atomic replacement of podcast.log reduces the risk for losing the contents
in case something goes wrong, especially if the process is terminated at the
wrong time, but also in case you happen to run two sessions concurrently.

For the latter case, a dynamic name for the temp log also helps.

I have lost podcast.log two times already this month. I am not sure why, but
I am hoping this makes the problem go away.