Verify the provided toolchain and host triple on install
Verify the provided toolchain and host triple on install

When rustup-init is running, we need to verify that the provided
host triple and toolchain channel can be used together to determine
a full toolchain triple (quadruple?).  If we can't do this during
the sanity checks, error out early.

inejge/rustup.rs,nodakai/rustup.rs,rust-lang/rustup,nodakai/rustup.rs,rust-lang/rustup,inejge/rustup.rs,nodakai/rustup.rs,rust-lang/rustup,rust-lang/rustup,inejge/rustup.rs,rust-lang/rustup,inejge/rustup.rs,inejge/rustup.rs,nodakai/rustup.rs,nodakai/rustup.rs