don't display capabilities without user explicitly asking for it

[leif]

Displaying secrets on the screen is undesirable if you aren't sure your screen isn't being viewed by a hostile camera.

There are several tahoe CLI commands that do this, all of which are commands that I'd like to be able to run without displaying capabilities:

• list-aliases
• put
• backup -v

Without -v, there is no indication of the backup command's progress. #1748 is the ticket about adding a progress bar, but it would also be useful to be able to use -v to see what is going on without printing secrets to the screen.

I suggest that all of these commands should get a new --show-caps option, and should not show capabilities when that option is not used.