Misc improvements

[mupuf]

Thanks for making this project! Just wanted to let you know I made the following changes to it which you may be interested in.

I'm willing to rework the commits, if you have some feedback on them.

[cherti]

Thanks for the improvements and sorry for taking so long to take a look at them! I do like the changes made so far, most of them do look useful.

I am however still thinking about https://github.com/cherti/socket-activate/pull/1/commits/73b6194e7d8df1acee403bf28b441119c761cdc2 (fix execution when to timeouts were set), which moves the timeout == 0 check from main() into unitCtrl, making unitCtrl the full controlling part, but I feel that this makes the code less stringent, because in this sense if timeout == 0, no actual terminateWithoutActivity happens, but the coroutine is running (predictably doing nothing).

So I think it's more elegant to not start it in the first place, so I would revert this commit, but the rest of the commits look fine and useful, thanks! :) (unless there is a scenario in which the timeout placement actually causes a problem that I'm missing, in which case please let me know! :))