Make ConfigurationOptions officially public.

Closes #1355.
Move `drb_argv_for` into the class where it belongs.
Rename `DrbOptions` to `DRbOptions`.

All the stdlib DRb stuff uses the "DRb" casing,
so we should follow that.
Rename `DRbCommandLine` to `DRbRunner`.

As in 7c0226046c039cc156cb615ac847ad2459d61422,
I don't feel like `CommandLine` describes
what the class did. Runner makes more sense.
Declare DRb classes private.

There's no good reason for these to be public.
As far as I can tell, there is no external code
using them. I did a github search and couldn't
find anything, and looked in detail at the one
project I expected to use them (spark) and it
does not.
Implement the last pending spec.
Cleanup some more docs.
Merge pull request #1457 from rspec/make-ConfigurationOptions-public

Make configuration options public