Rather than reusing `HOMEBREW_NO_AUTO_UPDATE` to indicate that we've
already made a check. This allows `HOMEBREW_NO_AUTO_UPDATE` to be
displayed properly by `brew config`.
Exclude the boring ones and exclude some that are already printed that
are just using their default values. HOMEBREW_PREFIX is the only one
where we’re always interested in the output to quickly communicate it.
Now that Python has been renamed it's important that we still output
which Python they are using for debugging purposes e.g.
```
Python: /usr/local/opt/python/libexec/bin/python => /usr/local/Cellar/python/2.7.13_1/Frameworks/Python.framework/Versions/2.7/bin/python2.7
```
Not quite a mass replacement as I've used OS X and Mac OS X where
describing specific older versions and added compatibility methods
for things in the DSL.
Due to our SCM wrapper in `Library/ENV/scm/git`, lookup is a bit more
complicated than just picking the first match in `PATH`. Make debugging
easier by printing the version and path of the Git actually used by us.
