xmlcatalog: restore ability to query system catalog easily

I've noticed that easy way of locating a DocBook XSLT in a configure
script of another project doesn't work anymore.  It is using something
like: xmlcatalog "" ${DOCBOOK_XSL_URI}/${DOCBOOK_XSL_PATH}.  The script
is then forced to a plain, suboptimal search using find utility.
Indeed, I retrospectively realize that the check was working just
by mere accident given that the window this presumably side-effect
was applicable had not lasted long, some 8 months between

  054c716e

introducing xmlInitializeCatalog in the LIBXML_TEST_VERSION-rooted
call chain, and

  f65128f3

reverting that again.

So while one can state /etc/xml/catalog constant explicitly, in
some use cases (such as the mentioned one -- that's why I wanted
to omit stating the full path in the first place), this is rather
an implementation detail, perhaps subject to change from that POV.
Therefore I propose to restore that behaviour in the targeted
manner without global disruptions this time around.

As a side-effect, this fixes indenting of the affected part.