sftp server, file browser

configuration
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to configure sftp server:
* add `[sftp]` section to `default.ini`
* add `root` option to `[sftp]` section, pointing at root folder for files

to enable uploads:
* create `__uploads__` directory in sftp root

to enable file browser:
* add menu option for `gosub('files')`

things to add
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* `FILE_ID.DIZ` caching, but that seems like premature optimization at this point
* ability to enter manual descriptions for files (will depend on same storage mechanism that holds `FILE_ID.DIZ` cache)
* virtual `FILES.BBS` generation so that sftp clients can view descriptions of a folder's files without being logged in via terminal server
* integration with x/84's IPC logging mechanism
* better detection of file download completion (currently, file is removed from user's tagged list as soon as it is opened via sftp, rather than when it is finished)
* tracking uploads/downloads (i.e., who uploaded what, how many times has something been downloaded, who downloaded what, etc.)
flagged is good.

I do plan to go with "tagging" for files -- loosely, I'd see a "pirate bbs" to allow:

- haliphax makes new group tag, "warezg0dz"
- haliphax marks group tag "member-private"
- haliphax invites dingo
- dingo uploads "Heroes of Might and Magic III.rar"
- dingo tags this file "warezg0dz"
- only dingo and haliphax may see the file

Allows users to create their own public and private "file conferences" without sysop access. That's the idea i have in store, anyway.