Add an animation to comfort the user

Sometimes msfconsole takes a little while to start. See #3725

This adds a fairly common ASCII spinner to the startup sequence, mainly to comfort the user that their machine isn't locked and things actually are happening while they're waiting to get hackin'.

I haven't spec'ed it, and the code organization isn't great, and there may be problems with printing to `$stdout` in the first place, so consider this PR more of a cry for help than something immediately landable.

That said, it works for me.
Yeah i'm not thrilled with it. On the one hand, you can get a sense (programmatically) if you've hung on the exit -- like if you typed exit when you should have typed exit -y in your resource script.

On the other, you will always return with exit status 0 even if something went wrong with the shutdown. Maybe you always were anyway.