Enhance JavacTool.getTask for flexibility

The public method JavacTool.getTask calls onto a similar private method that allows a Context to be specified. With that in mind, look at these lines

            if (out == null)
                context.put(Log.errKey, new PrintWriter(System.err, true));
            else
                context.put(Log.errKey, new PrintWriter(out, true));

1. If out is null, it should not overwrite the value in the context if that value is already set.

2. If out is non-null, and is a PrintWriter, it should not be rewrapped in another PrintWriter.