Originally, a raw call to the `git.config` state was used to disable
Homebrew Analytics. However, this is fragile and recently broke with the
Homebrew 1.0.0 release, which migrated the Homebrew repository location.
Now that we have support for custom modules, add a `homebrew` execution
module to allow running arbitrary Homebrew commands, and a
`homebrew_analytics` module on top which allows robustly disabling
Homebrew Analytics by using the builtin `brew analytics` command.

The execution module is named `homebrew` because the built-in module
[`mac_brew.py`] is not available (hidden behind the `pkg`
__virtualname__), so its methods cannot be called.

When running builds that are not from scratch, it is possible that
upstream changes have caused old revisions on master to stop building.
Because homu maintains the invariant that each merge to master is
passing, and we are only building the old revision to create a baseline
to test the new revision, it's safe to ignore failures in the old rev.

Installing git via Homebrew now requires XCode 8.
Additionally, this configures an OS X 10.11 image on Travis, which
matches what we have deployment in production (the default is an OS X
10.9.5 image on Travis).

With OS X 10.11, the previous launchctl.py module that we had backported
has been superseded by a new mac_service.py module in 2016.3.0.
Hence, backport the new mac_service.py module as well.

saltfs-migration: Ensure that our OS X builders are all running OS X
10.11 and have XCode 8 installed.

For some reason it seems Salt is not invalidating the `_modules` cache,
causing build failures on upgrade builds (i.e. the second build on a
not-from-scratch run). Invalidate the cache manually.

Note that `nullglob` is set for safety, but we always expect `rm` to
find cache files to invalidate. In case it does not find any files, the
glob will expand to nothing, which is considered an error, causing
failure via `errexit`. That is, not ignoring the result of rm is
intentional. (`nullglob` is nicer than `failglob` because it is
friendlier to loops over globs.)

Homebrew recently changed their packaging of Salt and Python, using a
virtualenvs for Salt. Because we were pinning an old version of Salt,
but not the rest of Homebrew, this caused problems where pip was not
being resolved properly. Update Salt to the latest version to fix this.

As part of the update, also update some states to avoid deprecated
parameters in favor of newer ones:
- archive.extracted: archive_user -> user, group
- cmd.run: user, group -> runas

Note that due to saltstack/salt#36552,
`archive.extracted` states still have the archive_user argument to
ensure parent directories are made with the correct permissions.

saltfs-migration: Upgrade Salt on all machines. Steps:
- Stop all salt-master and salt-minion services. Because we're upgrading
  Salt by two major versions at once, there are no guarantees that the
  masters and minions with different versions will be able to
  communicate.
- Use the install script to re-install Salt on each machine, the master
  first then the minion.
- Restart the salt-master and salt-minion services; pay attention to the
  system logs in case of startup failure.
- Do NOT run a highstate until all minions and masters have been updated
  to Salt 2016.3.0.

The install_salt script may be called again to reinstall or update Salt.
Because we are Salting our configuration files, force apt-get to use any
existing configuration files and ignore updates in the Salt packages.

OpenSSL is a "keg-only crate", which Homebrew 1.0.0 won't link because
```
we may end up linking against the insecure, deprecated system OpenSSL
while using the headers from Homebrew's openssl.
```

Instead, use the `OPENSSL_INCLUDE_DIR` and `OPENSSL_LIB_DIR` to
explicitly pass the paths to the Homebrew OpenSSL. Note that these paths
are currently hardcoded because they are unlikely to change.

For some reason, Homebrew was reporting this error on its stderr
but Salt was not picking it up as and failing the state - possibly
because the exit status seems to have been 0.

saltfs-migration: Run `brew unlink --dry-run openssl` on the Mac
builders, then `brew unlink openssl` once confirming the output is as
expected. This will unlink openssl on the existing builders.

If a different version of Salt is already installed via Homebrew, it
will not allow installing a different version without previously
unlinking the old one. Ideally there would be an atomic operation to do
both of these in one command, but it appears Homebrew does not have one.
Thus, users of this script should be careful to watch the output in case
the script is interrupted and no Salt installation is left linked.

Use an idempotent cmd.run to tap `homebrew/versions` only if necessary.

Homebrew 1.0.0 apparently tightened its return codes and will exit with
a non-zero status if linking fails during installation; there is no way
to install without linking or install and link --overwrite in one
operation. This makes it difficult to install multiple versions that
have conflicting links from the CLI, so add a custom script to handle
ensuring that autoconf and autoconf213 are installed and linked
properly, with autoconf's links over autoconf213's links,
in an idempotent way and doing as little as possible.