nixos: Split paras by \n\n in option descriptions

What annoyed me for a long time was the fact, that in order to break
into a new paragraph, you need to insert </para><para> in the
description attribute of an option.

Now we will automatically create <para/> elements for every block that
is separated by two consecutive newlines.

I first tried to do this within options-to-docbook.xsl, but it turns
out[1] that this isn't directly possible with XSLT 1.0, so I added
another XSLT file that postprocesses the option descriptions that are
now enclosed in <nixos:option-description/> by options-to-docbook.xsl.

The splitting itself is a bit more involved, because we can't simply
split on every \n\n because we'd also split text nodes of elements, for
example:

  <screen><![CDATA[

    one line

    another one

  ]]></screen>

This would create one <para/> element for "one line" and another for
"another line", which we obviously don't want because <screen/> is used
to display verbatim contents of what a user is seeing on the screen.

So what we do instead is splitting *only* the top-level text nodes
within the outermost <para/> and leave all elements as-is. If there are
more than one <para/> elements at the top-level, we simply don't process
it at all, because the description then already contains </para><para>.

https://www.mhonarc.org/archive/html/xsl-list/2012-09/msg00319.html



Signed-off-by: aszlig <aszlig@nix.build>
Cc: @edolstra, @domenkozar