Add `no-self-update` feature

[brson]

For distro packaging rustup self updates make no sense. And currently cause mysterious errors. Add a cargo feature, no-self-update that makes rustup update not do a self update and rustup self update print "error: self-updates are disabled for this build of rustup. If you installed rustup from a package manager, then use it to update rustup."

[steveklabnik]

Historically, with Debian, rubygems update --system was patched out of RubyGems by debian, causing endless strife between the two camps and a "never install ruby through apt" mentality by Rubyists for years.

rustup is not as critical as rubygems is, but I just wanted to make sure that you're ready for people to be using two-year old versions of rustup....

[brson]

@steveklabnik That's a good point. It's conceivable that rustup could detect when it has a system installation and do its own self-update in a way that shadows it.

I basically think that people should not be using package managers to install rustup (the upstream installation is supposed to be dead-simple), but packagers gonna package.

Anyway, if it causes problems we'll deal with them. This is fixed.