Display RUSTUP_HOME in rustup show and rustup show home

[fisherdarling]

This PR patches rustup show to also display the RUSTUP_HOME path:

$ rustup show
Default host: x86_64-unknown-linux-gnu
rustup home:  /home/fisher/.rustup

It also adds a new subcommand, rustup show home that directly prints the path:

$ rustup show home
/home/fisher/.rustup

This closes #1627.

[fisherdarling]

cc @kinnison

[kinnison]

Needs a teeny bit of cleanup, and then needs a test to verify that it works properly.

[fisherdarling]

Since rustup show always prints the home directory, I created another macro for_host_and_home! that would also get the config's rustupdir. If we should do this some other way, let me know.

[kinnison]

This is starting to look good -- please fold the formatting fixes into the commits which are relevant, and fold the fix to what you originally submitted into the top commit, so that there are two commits - one for the feature, and one for the test updates.