The helper is just bytes->str conversion with errors=backslashreplace,
which we can use for more than just paths.

We don't need a read-write key: we can just SSH directly to
jim-backups@backup.jim.sh instead and run commands thay way.
Remove read-write key and document it in the README.

Also add some tools to update the README variables on updates.

Automatically prunes after backup, although this doesn't actually
free up space (because we're in append-only mode).

Now individual files or patterns can have their own maximum sizes.

Force-include is a misnomer because it won't include files
that weren't considered at all (like files in an excluded subdir).
Instead, call it "unexclude" to make it slightly clearer that this
will just override the exclusions.

It clutters up the output and isn't super useful

This prevents it from e.g. needing a specific glibc version on the
client.

_thread.interrupt_main will trigger KeyboardInterrupt in the main thread.

This also ignores the "file changed while we backed it up" error, because
that isn't important enough to warrant sending an email.

We specifically don't want to catch BrokenPipeError; just list
file-related ones that we might expect to see if we hit bad
permissions, disk errors, or race conditions.