Rename rustup-setup to rustup-init

[brson]

The name setup has special meaning on Windows. It expects setup to be a windowsy setup program, doing specific things that rustup does not do. Let's avoid the whole problem by renaming the installer yet again to rustup-init. Anybody else have better names?

cc @vadimcn @retep998 what do you think?

[retep998]

Alternatively we could make it an actual installer.

[brson]

@retep998 That's the subject of #253

[taralx]

Alternatively you could embed a manifest.