Allow multiple vault password files

This PR adds support for passing multiple --vault-password-file. When decrypting, all the files will be used in turn until one matches.
I tried not to break vars_plugins (which are passed the current vault password) in case there's 0 or 1 password, but they will break for >1 (they'll get a list in `vault_password`).

I tried to change v2/* too. `make newtests` passes, but I don't really know how to make sure the v2 changes work.