id fixup with prefix

[David Cramer <david@thingbag.net>]

I'm adding support for the id fixup mechanism [1, 2] in the
clouddocs-maven-plugin [3]. To my mind, the most common use case (aside
from no fixup at all) will be to specify a prefix with global linkscope:

    <xi:include
        href="foo.xml"
	db:idfixup="prefix"
        db:idprefix="second-"
	db:linkscope="global"/>

The example xslt already treats global as the default linkscope [1]. If
the user specifies a db:idprefix attribute, it seems redundant to need
to specify db:idfixup="prefix" as well. I'd like to make it so that you
can just specify the db:idprefix, omitting db:idfixup since that's
implied already:

    <xi:include
        href="foo.xml"
        db:idprefix="second-"/>

Likewise, it should be an error to set db:idfixup to none, strip, or
auto and also to set a db:idprefix. Does that make sense?

Regards,
David

[1] https://github.com/docbook/transclusion-fixup
[2] http://docbook.org/docs/transclusion/2010-12-09/transclusion.html#d6e179
[3] https://github.com/stackforge/clouddocs-maven-plugin