Previously, for an XML like this:

    <sharedItems>
        <d v="2017-07-10T09:11:02"/>
        <d v="2017-07-10T09:11:03"/>
        <m/>
    </sharedItems>

the call like

    int pos = getXPathPosition(pDoc, "/x:sharedItems", "absent");

gave 3. That could result in mistakes, when a test would assert
on position "3" for a child element which name is mistyped.

I made such a mistake when creating a unit test trying to assert on
a position of the last element, and writing its name as "x:m", like
in rXPath itself; the return was 3, and I initially wrongly assumed
that the return is 1-based (like in xpath bracketed expressions).

rChildName made const OString&, for consistency with rXPath, or with
rAttribute in getXPath: child name is just a part of a longer xpath.

Change-Id: I7ba9c4466c75b1b10fce1ccf26ef3b56b4e11e87
Reviewed-on: https://gerrit.libreoffice.org/71614
Tested-by: Jenkins
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>