Various changes to implement building and installing docs to be compatible

Various changes to implement building and installing docs to be compatible
with the new "user-manual" settings in config from Roland. Docbook does not
seem to like dealing with more than one css file, so workaround that here.
Change 'make install' so it provides p_doc.css in the user-manual doc
directory so that functions well, and lastly modify 'make install' so that the
PATH is automatically set, and the 'user-manual' directive should done during
the install.