Combine drb_command_line and drb_options into one file.

These classes are related and will either be used
together or not at all. Seems better to just have
one drb file.
Add doc comment to get back to 100%.

Not sure why this is suddenly needed...
Merge pull request #1438 from rspec/drb-reorg

Combine drb_command_line and drb_options into one file.
Improve docs some more.
Move methods only needed by our tests into spec_helper.
Remove warning detection logic that is in rspec-support now.
Merge shared examples collection logic into main shared examples file.
Move shared group registry instance into world.

It belongs there, as that's where global
non-config state belongs.
Rewrite shared example group scoping specs.

The old specs were coupled to the implementation
(the Collection class), inhibiting refactoring.

As part of this I discovered a bug (see the pending
specs), which I'll fix in the next commit.
Fix bug with shared example group scoping.

Given the following contexts:

# main
describe "Parent" do
  # parent
  context "nested" do
    # child
    include_examples "some shared context"
  end
end

...the lookup algorithm for a shared example group
looked through the following contexts in this
order:

parent -> child -> main

...but it should happen in this order:

child -> parent -> main
Refactor: simplify how shared example groups are found.
Make `Registry#shared_example_groups` private.

No need for it to be public.
Update code and docs to make arguments more explicit.
Access `caller` lazily in a block rather than as an arg.

`caller` can be slow so it's best not to call it if
we don't need it (via `CallerFilter.first_non_rspec_line`).
Remove extra aliases for creating shared example groups.

We don't need `share_examples_for`, `shared_examples_for`,
`shared_examples` and `shared_context`.

Just `shared_examples` and `shared_context` are needed.
`shared_examples_for` has been around long enough and
reads well enough that we are keeping it, too.

A 2.99 deprecation is needed for this.
Update shared example group docs.
Add changelog entries.
Merge pull request #1425 from rspec/cleanup-shared-examples-code

Cleanup shared examples code (and a few other things)
Set $VERBOSE immediately.

This ensures that files loaded by `--require`
will have the flag set so that anything that
triggers a ruby warning will get printed as
expected.
Rename boolean flag so it's a predicate.

Still needs a 2.99 deprecation.