Add a skeleton FreeBSD specific glossary, and the appropriate glue to the
handbook to allow it to be incorporated in the build. This is not on by
default at the moment, and users who want it will need to set WITH_GLOSSARY
to a non-empty string when building ("make WITH_GLOSSARY=yes" being the
common case).

It is expected that committers will help to flesh out the glossary before
it is connected to the build by default; the current entries have been chosen
to illustrate the way that DocBook allows for "See also" and "See" in the
glossary elements.

This commit also takes the index out of the appendices in the handbook, as
this is not the usual place for an index to sit in a book, and moves it to
the end of the book.