Bring output of rustup show active-toolchain and rustup default into line with rest of rustup

[das-sein]

This resolves: #1406, and brings into line the fix for #1633

#1406 seems to just be the result of the rustup show active-toolchain command not producing the same output as rustup show, which includes the override reason. This has been changed so that if active-toolchain returns an override, it states the reason for the override. The change to rustup default is just to bring its output into line with the rest of rustup in using the "<toolchain> (<status>)" format.

Previous behavior of rustup show active-toolchain while nightly override is set:

canis@latrans:~/rustup.rs$ rustup show active-toolchain
nightly-x86_64-unknown-linux-gnu

New behavior:

canis@latrans:~/rustup.rs$ rustup show active-toolchain
nightly-x86_64-unknown-linux-gnu (directory override for '/home/canis/rustup.rs')

[kinnison]

This looks good though I'd like to know why we don't have a test affected by half the patch?

Is there an opportunity to add a test for rustup default ?

[das-sein]

@kinnison Good point! I did also think it odd, but didn't want to add an unnecessary test in case it was intentional. I'll work on some.

[das-sein]

@kinnison Test for show-active with override added.

[kinnison]

LGTM, thanks for the test.