Rather than relying on a `HOMEBREW_FORCE_BOTTLE` variable (which ends
up doing silly things like forcing bottle usage even when options are
provided) instead handle this at the `or_later` bottle detection
level so on prerelease versions of macOS any bottle looks like an
`or_later` bottle (unless various environment variables are set).

Fixes issues noted in:
https://github.com/Homebrew/brew/pull/4520#issuecomment-407229605

This matches the behaviour described in the test and on macOS.

Fixes #4459.

Ensure patchelf is installed to pour bottles and build bottles.

append_to_cccfg properly appends values to HOMEBREW_CCCFG variable

And be even more explicit whose problem it is when things are broken.

Share the regex where possible and otherwise ensure they are identical.

1. Running `brew linkage some_package` does not set the cache.
2. Running `brew linkage --cached some_package` when `DatabaseCache.empty?` returns `true` should build the cache.
3. Running `brew linkage --cached some_package` when `DatabaseCache.empty?` returns `false` should use the cache.

Lazily load db of type `DBM` instance variable for `DatabaseCache` so the corresponding database file isn't created in the `.use` block for a `DatabaseCache`.

Made `DatabaseCache.new` private, and changes instances in code that call it to use `DatabaseCache.use` instead.

Use cache if `HOMEBREW_LINKAGE_CACHE` exists in `formula_cellar_checks` and `formula`. Make functions private in `LinkageChecker`.

Ensure that a non-zero exit code is set both for missing random dylibs
and random missing dependencies.

Additionally, while we are here, drastically trim down the public
interface for this class to the bare minimum and allow getting the
output from `display_test_output` as a variable.

Fixes issue mentioned by @ilovezfs in:
https://github.com/Homebrew/brew/pull/3940#issuecomment-383794520

No reason to have it in macOS class and keeps consistent output with
`brew info` on Linux.

The `cask` attribute doesn't make as much sense on Linux but can be
ignored there. The advantage of this change is that (like #4086) it
allows figuring out the relevant cask for a formulae requirement on a
Linux machine.