Untabify DSSSL and XSL files and add to check-tabs target

Like with SGML files, using tabs in these files is confusing and
unnecessary.

doc/src/sgml/Makefile

@@ -357,7 +357,7 @@ endif # sqlmansectnum != 7
 
 # tabs are harmless, but it is best to avoid them in SGML files
 check-tabs:
-	@( ! grep '	' $(wildcard $(srcdir)/*.sgml $(srcdir)/ref/*.sgml) ) || (echo "Tabs appear in SGML files";  exit 1)
+	@( ! grep '	' $(wildcard $(srcdir)/*.sgml $(srcdir)/ref/*.sgml $(srcdir)/*.dsl $(srcdir)/*.xsl) ) || (echo "Tabs appear in SGML/XML files" 1>&2;  exit 1)
 
 ##
 ## Clean