Wrap main and remove os.Exit calls so all deferred functions will execute

[RebeccaMahany]

Relates to #1692

When we call os.Exit, deferred functions will not execute, which means e.g. the event log writer on windows won't get closed. For a more graceful shutdown, we wrap main as runMain and disallow os.Exit calls outside of the new main.

[directionless]

I think it's the right shape, but I wonder what we can do with these errors.

main doesn't have a logger, and stdout may not go anywhere. I wonder about a roundabout thing where runMain returns a slogger, and and error, and if the slogger isn't nil it can log the error... Or we can make sure all the error returns log first.

[directionless]

I didn't test this, but it looks okay

[directionless]

I think I'm game. I don't think I was that confident when I wrote service manager stuff, so this feels like a reasonable upgrade.