[Ovmsdev] Command prompt in secure mode

At Mark's request I have implemented a change to the command prompt after switching to secure mode:

OVMS > ena Password: Secure mode OVMS #

As implemented in this commit, the color also changes from green to red, but that is only a proposal. I also looked at blue, but on my terminal it's not so distinct from black. If preferred, we can have both modes be green.

Actually, as I thought about this, I wondered whether our console welcome banner and prompt should be different from green since all the info-level log messages are green. But there are not many ANSI color choices and they don't look the same on all terminals, so we can't get carried away with this.

                                                    -- Steve