keep Cygwin docs well-formed and valid

[Joshua Daniel Franklin <joshuadfranklin at gmail dot com>]

I just did a commit that touched a lot of sgml files. Now both the
User's Guide (cygwin-ug-net.sgml) and API Reference (cygwin-api.sgml)
are well-formed and valid DocBook XML. The current SGML tools still
work fine with it, but I hope to move the doc Makefile to xmlto soon
to end the confusion with different SGML toolchains (as far as I know
xmlto is standard across platforms, and it's already packaged for
Cygwin).

For anyone who edits the documentation, though, please keep the files
well-formed and valid! This means:

--All tags should be in lower case (like <orderedlist>, not <oRdErEdLiSt)
--If you open a <para>, close it with </para>
--Use entities for 8-bit characters, like &copy; for © and &reg; for ®.
--Close empty tags, though note that <xref> is weirdly NOT empty but
should be used like this: <xref linkend="cygpath"></xref>
--Always put tag attributes in quotes

Thanks.