Add client command for switching to previous user

[Fayti1703]

I've often run into the situation where I was on a user for a while, then had to swap to another user to xfer some GC or upgrades to the first user, and then immediately swap back to continue doing things.

It'd be nice if that swap back didn't involve having to type out the first user's name.

Commonly, CLI programs use - for this -- e.g. if you cd somewhere, cd - goes back to the previous directory you were at. (git checkout supports this too).

Since - isn't a valid username, the client could accept user - as a command to switch to the previous user.