DocBook: Fix non-determinstic installation of duplicate man pages

Some kernel-doc sections are included in multiple DocBook files.  This
means the mandocs target will generate the same manual page multiple
times with different metadata (author name/address and manual title,
taken from the including DocBook file).  If it's invoked in a parallel
build, the output is non-determinstic.

Build the manual pages in a separate subdirectory per DocBook file,
then sort and de-duplicate when installing them (which is serialised).

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
[jc: fixed conflicts with the docs tree]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>