panc: Deprecated should always return boolean true

This allows us to start removing the hundreds of instances of `... with {deprecated(...); true;}` in the schema.

It's not clear why it ever returned undef, but it appears to have been this way since the original rewrite in Java and looks like it was originally default boilerplate for new functions.
The documentation and comments claim it should be returning the string passed to it, but this has never been the case.

panc-docs/source/standard-functions/standard-functions.rst

@@ -274,16 +274,14 @@ deprecated -- print deprecation warning to console
 Synopsis
 --------
 
-string **deprecated** (long *level*, string *msg*)
+boolean **deprecated** (long *level*, string *msg*)
 
 Description
 -----------
 
-This function will print the given string to the console (on stderr) and
-return the message as the result, if ``level`` is less than or equal to
-the deprecation level given as a compiler option. If the message is not
-printed, the function returns undef. The value of ``level`` must be
-non-negative.
+This function will print the given string to the console (on stderr), if
+``level`` is less than or equal to the deprecation level given as a compiler
+option and always returns true. The value of ``level`` must be non-negative.
 
 .. _dict:
 

panc/src/main/java/org/quattor/pan/dml/functions/Deprecated.java

@@ -29,19 +29,18 @@ Centre National de la Recherche Scientifique (CNRS).
 import org.quattor.pan.dml.data.Element;
 import org.quattor.pan.dml.data.LongProperty;
 import org.quattor.pan.dml.data.StringProperty;
-import org.quattor.pan.dml.data.Undef;
+import org.quattor.pan.dml.data.BooleanProperty;
 import org.quattor.pan.exceptions.EvaluationException;
 import org.quattor.pan.exceptions.SyntaxException;
 import org.quattor.pan.ttemplate.Context;
 import org.quattor.pan.ttemplate.SourceRange;
 
 /**
  * Prints the argument to the standard error stream if the given level is less
- * than or equal to the deprecation level option. The returned value is the
- * message, if the message is printed; undef, otherwise.
- * 
+ * than or equal to the deprecation level option. The returned value is always true.
+ *
  * @author loomis
- * 
+ *
  */
 // FIXME: This method should really only accept one argument now.
 final public class Deprecated extends BuiltInFunction {
@@ -91,7 +90,7 @@ public Element execute(Context context) {
 
 		assert (ops.length == 2);
 
-		Element value = Undef.VALUE;
+		Element value = BooleanProperty.TRUE;
 
 		if (!context.isCompileTimeContext()) {