Tidyup patch for Command/CommandLine and classes that use it

This patch changes some core APIs as follows:

• A CommandLine object is no longer an Iterator<String>. Instead, it implements Iterable<String> so that a client don't have to worry about reentrancy when iterating
  the arguments. In addition, there is a new method for iterating the argumens as Tokens, and new constructors for building a CommandLine from command and argument Tokens.

• The (optional) AbstractCommand is an abstract base class for classes that implement the Command interface. This provides an execute(String[]) method that simplifies the stereotypical implementation of the "maim" entry point method, and an exit(int) method.

The patch changes a large number of command classes to use the modified APIs; e.g. to extend AbstractCommand, and call the exit method to indicate command failure.

The patch changes the way that the interpreters handle redirection and pipe streams. (The interpreters now use StreamMarker objects to denote certain distinguished streams. These are mapped to the corresponding real stream by the invokers. This makes it easier to get the stream opening / closing logic correct ... especially in bjorne.)

The patch changes the way that command return codes (e.g. from the "exit" method) are handled internally by the CommandShell infrastructure. This relates to a bug; see below.

Finally, it incorporate fixes (and non-fixes) for a couple of problems that Levente spotted.

• There is a tweak in to ConsoleCommand that will warn the user that System.in,out,err will NOT work as expected from a "console -n" shell if you are using the "default" or "thread" invoker. This tweak is a short term hack. IMO, the long term solution is to get rid of these invokers entirely ... but that's a different topic. (The proclet mechanism used by "proclet" invoker makes these streams almost work as expected ... and an invoker based on isolates would fully address the problem.)
• There is a fix for a bug that stopped "internal" commands from running. The way that I was handling return codes was broken for "internal" commands.