See discussion in Homebrew/brew#800.

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

Fixes #796.

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

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

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 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.

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.

Otherwise it can end up as e.g. `bin/git` which then breaks when we
`cd` to another directory and try to run it.

Go makes extensive use of weak imports so we need to allow them when
building Go-using software.

Closes https://github.com/Homebrew/homebrew-core/issues/4047.

Also, check in this manpage, adjust the Rakefile and remove the old one.

Fixes #731.

Flips HOMEBREW_RUBY_MACHO to HOMEBREW_NO_RUBY_MACHO.