Go back to using hash as base name for resume and torrent files

Go back to using hash as base name for resume and torrent files

The format introduced in TRAC-394 is nice but brings its own issues (e.g.
TRAC-4189, #122). I'm okay with a bit of grepping myself if it makes the
experience better for end users.

Using hashes results in filenames well under 255 bytes limit on some
filesystems. If even that will not be enough, I'd suggest reporting the
issue elsewhere.

Fixes: #122