doc: use consistent id's when generating html files

The html documentation is generated using a2x which calls docbook tools
to do the work.  The generate.consistent.ids parameter ensures that when
the docbook stylesheet assigns an id value to an output element it is
consistent as long as the document structure has not changed.

Having consistent html files reduces frivolous changes between builds.
Distributions can more easily deploy multiple architecture builds and
compare changes between package versions.  End-users avoid needless
changes in files deployed or backed up.

The generate.consistent.ids parameter was added in docbook-xsl-1.77.0.
Older versions gracefully ignore the parameter, so we can pass the
parameter unconditionally.  Most distributions contain docbook-xsl newer
than 1.77.0.  This includes Fedora, Debian, Ubuntu, and RHEL/CentOS 7.
RHEL/CentOS 6 and Debian Wheezy (old stable) ship with an older version,
unsurprisingly.

Signed-off-by: Todd Zullinger <tmz@pobox.com>