Provide a method to execute an external command without using a shell

The Exec task always spawns a shell to invoke the external command. This is convenient, but non-portable, insecure (though not exploitable here), and requires non-portable escaping.

A better solution is to provide a way to execute an external command without using a shell. It should have the following properties:

• Lists are respected.
• Any character can be passed to an external command. No manual escaping required.
• Special characters in property values do not cause problems.
• No shell is invoked, unless the external command is the shell.
• It should be possible to pass multiple arguments to the shell
• The syntax should be something other than requiring a seperate XML element per argument.