Updated the documentation building tools to work with a recent linuxdoc-tools package.
Note that this is (and always was) a terrible hack and we really should
replace it with something reasonable which wouldn't need changing every
time linuxdoc-tools evolve.
I also needed to include a patched version of LinuxDocTools.pm, because the
original one explicitly refused to work with a non-linuxdoc DTD. The authors
of linuxdoc recommend to use sgmltools-lite in such cases, but it would mean
rewritting our formatting rules to the DSSSL language which I don't dare to
speak about here :)