Created a new set of entities for manual pages.

The construct:

<citerefentry>
  <refentrytitle>foobar</refentrytitle>
  <manvolnum>1</manvolnum>
</citerefentry>

is a pain to type, and messes up the pretty-printing of the source code.
Replace every occurence of a entry like that with:

&man.foobar.1;

Adjusting the manual page name and section number appropriately.

The definitions for these entities are stored in man-refs.ent. This
file is in doc/share/sgml because it is not just specific to the Handbook.
I expect the DocBook'd FAQ and Tutorials (coming RSN) to use them as
well.

A new PUBLIC identifier has been created for these entities, and added to
the catalog file.