Version Control With Subversion Source

has been deprecated), move most of the Berkeley DB-specific
information to a new Appendix D and purge the book of the implication
that choosing a repository backend is really a thing.

* en/book/appd-berkeley-db.xml
New appendix, primarily populated with material culled from...

* en/book/ch05-repository-admin.xml
...this chapter, which now clearly favors FSFS.

* en/book/book.xml
Hook up the new appendix.

* en/book/ch03-advanced-topics.xml
* en/book/ref-svnadmin.xml
Update some cross references.