Add support for posting changes to Cliosoft SOS repositories.

Cliosoft SOS (https://www.cliosoft.com/products/sos/) is an
enterprise-grade source code management solution used throughout the
hardware industry. Its features are tailored to large, distributed teams
working with inter-dependent projects. It supports large files,
"packages" of related files that must be managed under a single
revision, general attribute metadata support, file locking, ACLs, sparse
checkouts, snapshots, and other feaures useful in enterprise settings.

Through a combination of RBTools 3.1+, Review Board 4.0.6+, Power Pack
5+, and SOS 7.20+, users will be able to configure Review Board to talk
to Cliosoft SOS repositories and post changes for review.

Users can post explicit selections for review, such as:

    $ rbt post select:-scm
    $ rbt post 'select:-sunm -sor -scm'

Or they can post SOS changelists (new in 7.20) by name:

    $ rbt post my-changelist-name

The generated diff uses the DiffX (https://diffx.org) format, with
metadata specific to SOS. This is used to identify the workarea, SOS
Project, SOS Server, changelist ID, and RSO (Revision Search Order) that
was posted.

Some of this information is also returned during diff generation for
storage in the review request. This enables custom SOS fields in the
review request to show up, which will contain information useful to
reviewers and the owner to help determine which workarea and changelist
contains the changes.

That logic (along with `get_tree_matches_review_request()`) will also
enable smart matching of review requests for `rbt post -u`, which is
another change in development.

At the moment, this does not support posting files bundled as SOS
packages for review. That is planned for a subsequent release.

SOS 7.20 is required for this release, though the code has been written
to not strictly enforce this version yet, as some customers are using
patched builds of earlier releases that contain this support. In time,
we'll want to make 7.20 a hard requirement.

This work is a collaboration between Beanbag and Cliosoft. A big thanks
to them for all the testing, support, and all the new functionality
added to SOS 7.20 to make this integration happen.

Testing Done:
All unit tests pass.

Extensively tested with a variety of types of changes in local SOS
workareas. This included:

* Selection-based posting (default and custom selections)
* Changeset-based posting
* Included files and excluded patterns
* Changed, added, and deleted files (referenced in selections and changesets)
* Symlink changes (though this support still requires further work, and based
  on discussions with Cliosoft will be moved to a future release)
* Verification that existing SOS selections are preserved after the
  operation completes.
* Tested posting with `rbt post -u` (when combined with in-progress changes
  for smarter matching)

Reviewed at https://reviews.reviewboard.org/r/12237/