ui: add special-purpose atexit functionality In spite of its longstanding use, Python's built-in atexit code is not suitable for Mercurial's purposes, for several reasons: * Handlers run after application code has finished. * Because of this, the code that runs handlers swallows exceptions (since there's no possible stacktrace to associate errors with). If we're lucky, we'll get something spat out to stderr (if stderr still works), which of course isn't any use in a big deployment where it's important that exceptions get logged and aggregated. * Mercurial's current atexit handlers make unfortunate assumptions about process state (specifically stdio) that, coupled with the above problems, make it impossible to deal with certain categories of error (try "hg status > /dev/full" on a Linux box). * In Python 3, the atexit implementation is completely hidden, so we can't hijack the platform's atexit code to run handlers at a time of our choosing. As a result, here's a perfectly cromulent atexit-like implementation over which we have control. This lets us decide exactly when the handlers run (after each request has completed), and control what the process state is when that occurs (and afterwards).