Eliminate erroneous role="directory" attributes from <filename> elements
and replace them with class="directory" [1][2]

[1] http://docbook.org/tdg/en/html/filename.html
[2] http://www.freebsd.org/doc/en/books/fdp-primer/sgml-markup-docbook.html#AEN1799

(4.2.5.5)

Approved by: trhodes, gabor (mentor)