support seeding completed read-only files

[milahu]

fix #136

based on https://github.com/ikatson/rqbit/commit/d980e295b2a9b90efe84b10115829d3774155224

alternative to #373

packaged in nur.repos.milahu.rqbit

building rqbit with rust 1.87 fails with

error[E0658]: `let` expressions in this position are unstable
  --> crates/librqbit_core/src/magnet.rs:41:16
   |
41 |             && let Ok(id20) = Id20::from_str(url)
   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information

this seems to work, but im getting these warnings

WARN librqbit::torrent_state::initializing: Error setting length for file "somefile.txt" to 12345: Os {
    code: 22,
    kind: InvalidInput,
    message: "Invalid argument",
} id=0 info_hash=xxxxxxxxx