XML fragment fixup for refsect1

[alfsb]

This PR changes the fix up for refsect1 to <title>_</title><simpara>%failed xi:xinclude%</simpara>. As this cannot be parsed as an XML document, code is also changed to support fix ups that are XML fragments.

This PR, php/doc-it#55 and doc-en patch below fixes doc-it building.

diff --git a/reference/gmp/gmp/construct.xml b/reference/gmp/gmp/construct.xml
index df2e9e0893..cdc32f70bb 100644
--- a/reference/gmp/gmp/construct.xml
+++ b/reference/gmp/gmp/construct.xml
@@ -18,9 +18,7 @@
  </refsect1>
 
  <refsect1 role="parameters">
-  <xi:include xpointer="xmlns(db=http://docbook.org/ns/docbook) xpointer(id('function.gmp-init')/db:refsect1[@role='parameters']/*)">
-   <xi:fallback/>
-  </xi:include>
+  <xi:include xpointer="xmlns(db=http://docbook.org/ns/docbook) xpointer(id('function.gmp-init')/db:refsect1[@role='parameters']/*)"/>
  </refsect1>
 
  <refsect1 role="seealso">

Two notes:

1. This is a small-scale example of the effects of removing <xi:fallback> from sources, as the automatic fixups can push/keep translations into building state.
2. I think I found a bug on PhD, while developing this.

The fixup <title></title><simpara>%failed xi:xinclude%</simpara> (empty title) is saved on .manual.xml as:

<refsect1 role="parameters">
 <title/><simpara>%failed xi:xinclude%</simpara>
</refsect1

and is rendered by php phd/render.php --format bigxhtml as:

<div class="refsect1 parameters" id="refsect1-function.bcdivmod-parameters">
  <h3 class="title"><p class="simpara">%failed xi:xinclude%</p></h3>
</div>

Instead of:

<div class="refsect1 parameters" id="refsect1-function.bcdivmod-parameters">
  <h3 class="title"></h3>
  <p class="simpara">%failed xi:xinclude%</p>
</div>

In other words, empty <title> causes their siblings to be rendered inside the title, instead of around it.

To workaround this, and to avoid rendering %failed xi:xinclude% as big as titles, I placed an _ inside de title.

I'm not so certain about %failed xi:xinclude%/_. Another alternative was to keep these elements empty, so XInclude failures does not inject warnings in rendered manuals, and would like comments about this.