Gatekeeper's Path Randomization is currently making automated
installation a nightmare. Let's manually toggle the (undocumented) app
translocation bit in the `com.apple.quarantine` extended attribute.
While we're at it, let's also toss in some fixes:
- zip downloads with improper permissions that prevent us from
quarantining
- quarantine/release/skip downloads as requested by the user
- In some cases (usually old CLT versions) Swift is available but needs an
extra guard to use the quarantine API.
- xattr's native filesystem recursion is an Apple extension which is
not available in Mojave; so let's use xargs.
- Insert a quarantine support check in brew cask doctor.
FixesHomebrew/homebrew-cask#51554, and fixesHomebrew/homebrew-cask#51538.