Added rudimentary authorization with `be serve --auth FILE`.

Special username 'guest' is not allowed to change name,password or
write to the repository.  All other users in the auth file are allowed
to do all of that.  A more robust solution would be to have POSIX
permissions on each storage item, or something.

Note that while the server supports name/password changes for
non-guest users, there is no command-line interface to this
functionality.  There is also no automatic way to register
(i.e. create entries).