Rustup should attempt a self update

Somewhat annoying, there was a breaking change in channel*.toml which my old rustup couldn't handle. It wasn't until asking in irc that I found out rustup had a 'self update' feature.

Not too much to gain and probably somewhat troublesome but rustup should be able to check for updates in case it runs into an error easily attributed to being out of date (potentially made easier by adding versioning and version check to the channel*.toml's or any other data passed to the rustup application that is version dependent).

The user shouldn't be bothered with this when they know they want to update and the rustup application being out of date is the issue. Or at least they should be made known of the 'self update' feature at times it seems appropriate.

Start by tracing how rustup handles channel*.toml errors and where the existing self update feature is exposed. Define the update-check conditions and user messaging with the maintainers, then add coverage for the chosen behavior; done means an appropriate outdated-data error can inform or trigger updating without disrupting explicit update requests.