Check for Ruby files in taps that are outside of the detected `Formula`
directory for a tap but inside one of the other potential directories.

This usually indicates a formula has been added in the wrong directory
in a tap and is used to fail CI in this case.

And tell people to delete the HOMEBREW_REPOSITORY one. The
HOMEBREW_PREFIX one is preferable for bottling.

Allow our Jenkins CI testing for Homebrew/brew to live outside of
`/usr/local` without complaint (as it's not using any binary packages
anyway).

This reverts commit 7f13b37b, reversing
changes made to 62e0ecda.

This reverts commit 89c33489, reversing
changes made to 7f13b37b.

Non-administrative users are unable to run `brew linkapps` (and possibly other
commands) under normal circumstances.

And, if it isn't, print more helpful debugging messages.

Fixes #1456.

Also: freeze the relevant arrays.

Add static libs and dylibs commonly put in /usr/local/lib by Symantec Endpoint Protection to the brew doctor white lists

Multiple issues have been filed about this environment variable, hence
give a warning. #932 might make this unnecessary when merged, but
hopefully this warning can be merged quickly.

Fixes #1223.

Update some messaging based on recent user confusion and the
`/usr/local/Homebrew` and Ruby 2 migrations.

This is no longer relevant in the bright, new tagged future.

Fixes #1151.

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.

But remove some manual `.freeze`s on constants that shouldn't be
constants.

These definitions are scattered throughout the codebase which makes it
hard to refactor them later (my goal is to move them outside of
HOMEBREW_LIBRARY). Unify their definitions for clearer code and easier
movement later.

This is used in diagnostic (and will be used in other places later)
rather than hard-coding other directories later.

Apple reset this on every OS X major (and some minor) updates and it
always proves a painful and unnecessary step. Instead just check the
directories we actually care about are writable.

This may mean if these directories do not already exist (although they
are now created by the installed) that `brew link` will fail and require
manual intervention but this seems to be superior for both new and the
majority of existing users.

Better use the abstraction layer so e.g. Linux could have similarly
fatal checks for these things.

Highlight that the main reason for preferring /usr/local is because of
bottles. There's been some confusion around this e.g.
https://github.com/Homebrew/homebrew-core/issues/3968

These are intentionally duplicated for now until the Cask migration is
complete.

Closes #694.