Upgrade rust-installer to v2
Install copyright information and package docs

This distributes docs in a separate package called rust-docs. The rust-packaging
project will combine it with Rust and Cargo into a single installer in a variety of formats.
mk: Make distcheck build binary tarballs on windows

These work, but aren't being built and uploaded because the bots run
'distcheck' not 'dist'.
mk: Package mingw components in unix installer on windows

This puts stdc++ and the unwinding dll into the main package
and creates a separate rust-mingw package for everything else.
Simplify some logic in dist.mk
auto merge of #20317 : brson/rust/rust-installer-v2, r=alexcrichton

This upgrades to a version of [rust-installer](https://github.com/rust-lang/rust-installer) that supports multiple 'components', so that the output can be combined into a [single installer](https://github.com/rust-lang/rust-packaging) that contains both Rust and Cargo (#16456).

At the same time it also packages documentation into a component called 'rust-docs', which will again be merged into the combined installer (#14916).

It creates yet *another* component, 'rust-mingw', to carry the optional parts of mingw necessary to link with Rust on windows, and makes distcheck build all these installer components for upload by the buildbots. It also moves the 'third-party' docs from the 'bin/' folder to 'share/doc/rust' where the rest of the docs live, temporarily removing them from the .exe installer.

This needs to be merged at roughly the same time as the [corresponding Cargo PR](https://github.com/rust-lang/cargo/pull/1102).

Next steps:

* Modify rust-packaging to produce combined .exe and .pkg installers.
* Set up automation for building rust-packaging every time a rust build is completed.
* Rename the 'rust' installer artifacts produced here to 'rustc' and the the 'rust-combined' produced by rust-packaging to just 'rust'.
* Remove the .exe and .pkg packaging from the main repo.

r? @alexcrichton p=1 please. There's a lot more to do here.