brew.sh: complain when running Homebrew as root.

As discussed: announce that we are deprecating running Homebrew as the
root user as it's insecure and requires disabling the sandbox.

Fixes #796.

diagnostic: tweak /usr/local messaging.

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

config: set TMPDIR, TEMP and TMP to HOMEBREW_TEMP

Improve HOMEBREW_BREW_FILE unset failure case.

Unify heredoc style.

Allow per-cask `cleanup`.

Print a better exception message and handle this in more cases rather
than producing confusing errors when it’s unset.

bin/brew: improve $HOMEBREW_REPOSITORY/bin/brew handling.

brew.sh: don't require /usr/local/Cellar creation.

update-report: also link `brew*.1` manpages.

Merge Cask’s `Pathname` extension with Homebrew’s.

Add vendored `plist` to ReadMe.

Otherwise if your `HOMEBREW_PREFIX` and `HOMEBREW_REPOSITORY` are not
equal then your tap manpages will be linked but your `brew*` ones will
not.

If `/usr/local` is the prefix but not the repository (so bottles can be
used) then running `/usr/local/bin/brew` works great but
`$HOMEBREW_REPOSITORY/bin/brew` assumes the prefix is
`$HOMEBREW_REPOSITORY`. This is a pain when doing work on the Homebrew
repository and having e.g. `bin` in your `$PATH`. Improve this
behaviour so `bin/brew` knows how to handle this situation.

This will enable this code to be reused in a future commit and makes it
more obvious what’s being done.

The previous approach of trying to do it on one line was confusing.

If you're using e.g. a `/usr/local/homebrew` prefix then don't require
the `/usr/local/Cellar` to be manually created to avoid e.g.
`/usr/local/homebrew/Cellar` being used. Let's do all we can to let
people use this `Cellar` location as it means they can put their
repository wherever they like and still use all our bottles.

TMPDIR, TEMP and TMP (when set) are not whitelisted for writing in
sandbox.rb, which could result in sandbox violations when programs
attempt to write to these locations.

Setting TMPDIR, TEMP and TMP to HOMEBREW_TEMP (which defaults to /tmp
when not set) works around the aforementioned problem and also improves
uniformity in the locations of tempfiles created during Homebrew
operations.

Caveat: Non-matching HOMEBREW_TEMP and TMPDIR could lead to undesirable
side effects in certain cases, e.g., emacsclient not being able to find
an existing server (whose socket lives in $TMPDIR/emacs$UID/) when
launched through brew edit.

Enable ruby-macho by default.

update: make GIT_EXECUTABLE an absolute path.

Move Cask’s vendored `plist` to `Homebrew/vendor`.

Make -no_weak_imports opt-in.

Revert "diagnostic: ignore duplicated `brew-cask` commands"

The experiment to add `-no_weak_imports` unconditionally has been a bit
of a failure. It's broken more than it's fixed and I don't think we can
have this as a default for Xcode 8.

Add `ENV.no_weak_imports` to be used by formulae authors and make
`ENV.no_weak_imports a no-op (for now).

We may reconsider this behaviour in future.