Make rustup.sh interactive and not require sudo

[brson]

This changes rustup in the following ways.

• rustup runs 'sudo' to do the install unless --disable-sudo is passed
• it displays an initial welcome message that explains what is going to happen, including running sudo, then requires confirmation to proceed
• the -y flag disables the welcome message
• adds some ugly metadata upgrade code to deal with old root-owned .rustup directories
• warns if running as root

The basic welcome message looks like this:

Welcome to Rust.

This script will download the Rust compiler and its package manager, Cargo, and
install them to /usr/local. You may install elsewhere by running this script
with the --prefix=<path> option.

The installer will run under 'sudo' and may ask you for your password. If you do
not want the script to run 'sudo' then pass this script the --disable-sudo flag.

You may uninstall later by running /usr/local/lib/rustlib/uninstall.sh,
or by running this script again with the --uninstall flag.

Continue? (y/N)

[brson]

r? @alexcrichton but we might wait a day or two for comment from https://internals.rust-lang.org/t/what-changes-are-necessary-to-make-rustup-sh-handle-sudo-itself/1948/5

[alexcrichton]

r+ from me, but I agree on waiting for a few comments

[steveklabnik]

I like the welcome message 👍

[brson]

I'm going to merge this, but not deploy until I've got a patch for travis in.

[brson]

Here's the patch to travis travis-ci/travis-build#443