Improve xmllint --dtdvalid, --relaxng, --schema and --schematron completions.

contrib/xmllint

@@ -17,8 +17,24 @@ _xmllint()
 			_filedir
 			return 0
 			;;
-		--path|--dtdvalid|--dtdvalidfpi|--maxmem|--encode|--pattern|\
+		--path|--dtdvalidfpi|--maxmem|--encode|--pattern)
-		--relaxng|--schema|--schematron)
+			# argument required but no completions available
+			return 0
+			;;
+		--dtdvalid)
+			_filedir dtd
+			return 0
+			;;
+		--relaxng)
+			_filedir rng
+			return 0
+			;;
+		--schema)
+			_filedir xsd
+			return 0
+			;;
+		--schematron)
+			_filedir sch
 			return 0
 			;;
 	esac