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

  054c716

introducing xmlInitializeCatalog in the LIBXML_TEST_VERSION-rooted
call chain, and

  f65128f

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.

xmlcatalog.c

@@ -312,7 +312,8 @@ static void usage(const char *name) {
     /* split into 2 printf's to avoid overly long string (gcc warning) */
     printf("\
 Usage : %s [options] catalogfile entities...\n\
-\tParse the catalog file and query it for the entities\n\
+\tParse the catalog file (void specification possibly expressed as \"\"\n\
+\tappoints the default system one) and query it for the entities\n\
 \t--sgml : handle SGML Super catalogs for --add and --del\n\
 \t--shell : run a shell allowing interactive queries\n\
 \t--create : create a new catalog\n\
@@ -408,11 +409,18 @@ int main(int argc, char **argv) {
 	    continue;
 	} else if (argv[i][0] == '-')
 	    continue;
-	filename = argv[i];
+
+	if (filename == NULL && argv[i][0] == '\0') {
+	    /* Interpret empty-string catalog specification as
+	       a shortcut for a default system catalog. */
+	    xmlInitializeCatalog();
+	} else {
+	    filename = argv[i];
 	    ret = xmlLoadCatalog(argv[i]);
 	    if ((ret < 0) && (create)) {
 		xmlCatalogAdd(BAD_CAST "catalog", BAD_CAST argv[i], NULL);
 	    }
+	}
 	break;
     }