The sftp accounts file could support aliases to make configuration easier

[exarkun]

The sftp accounts file includes a capability string for each credential being set up. The capability string identifies the sftp storage root for that credential.

The capability string currently must be an actual capability string. If it could be an alias (see tahoe create-alias) then it would be a bit easier to configure sftp accounts - both because each line in the accounts file could be shorter and because it allows some indirection.