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.)
It's just that it would say "0.5M" instead of "500K" if it's above 400K. That's all the 1024 * 400 is about. The multiplication isn't necessary, but is there for readability.