Add a way to mark commands as 'interactive-only' so they are ignored in downloaded scripts

Commands such as `c1 pt` and `sdi12 pt` will cause a Wombat to hang if they are run from a downloaded script because there is no way to issue the ctrl-D to exit passthrough mode. Other commands such as `c1 pwr` or `sd log` also make no sense in this context.

It would be good to have a way to either mark commands as being interactive-only and/or have a variable that says a command is being run as part of a downloaded script so the command code can decide whether to run or not.