CLI: add a way to change an existing alias without directly editing the aliases file

[davidsarah]

tahoe add-alias fails if the alias already exists, and so cannot be used to change an existing alias entry. There should be a command or option to do that (say, tahoe add-alias --replace or tahoe replace-alias), and the tahoe add-alias error message should say what it is.

There is also no command to delete an alias.

[daira]

Ramakrishnan ran into this problem; I had to tell him to edit the aliases file to remove an alias.

[warner]

Meh, I'm kind of in favor of just having people edit the file. I think we need at least one alias-manipulating command for the sake of discovery, but I'm not super-convinced that we need modification and removal commands.

That said, I wouldn't object if someone really wanted to add them. Just be cognizant of secret dircaps showing up in .bash_history and globally-visible 'ps' listings. You can create an alias without revealing it that way, but you can't add one safely. If we do want to enable modification, we might want to make the user paste in the new dircap via a raw_input() prompt. At that point it starts to look easier to tell people how to edit the aliases file :).