Make adding section's id to their class optional.

svn path=/trunk/tools/boostbook/; revision=84559
Use US-ASCII for single page html.

Same as chunked html. We use ASCII because using UTF-8 can go wrong when a
server is set up to specify another encoding (this does happen). `xsltproc`
will encode all non-ASCII characters as html entities, so nothing will be lost.

svn path=/trunk/tools/boostbook/; revision=84560