diagnostic: move some more macOS doctor checks.

Move some `brew doctor` checks that are pretty macOS specific so they
are only run on macOS.

Use Python's flock instead of Ruby's

Deprecate `--caskroom` flag.

Fix cli default options.

Vendor compatibility fixes

Relocate virtualenv orig-prefix

This works around older versions of `which`, which return a
string on failure.

Ruby first gained flock in 1.8.7, which is a problem since we're using
this lock utility in `vendor-install` in order to install a newer Ruby.

Fortunately, Python 2.3(!) has flock support.

Fix `uninstall` order.

Add `SUDO_ASKPASS` to `brew-cask` manpage.

Fix `previous_version_and_checksum` method.

Fix appcast checkpoint command.

Refactor `CLI`.

patch: clean up with Forwardable

audit: fix versioned alias errors in homebrew/core introduced in a7c6a57c