remove bbs.meta, reduce the usage of bbs.modify

moving towards a cleaner API
rewrite login

relies on authentication for login
no bbs:users anymore

sshbbsd uses keyboard interactive and will display a banner to indicate
that registering is done with user ".".
rkv.S, with sorted strings moved to rkv

this makes more sense if at last we decided to use a more complex
storage backend, which has some built-in data strucures.
built-in session for tagbbs