support seeding completed read-only files

[milahu]

fix #136

based on d980e29

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

[milahu]

this seems to work, but im getting these warnings

for some torrents, i also get these errors

error writing to file 0 (""some-file.txt"")

Caused by:
    0: error calling pwritev
    1: EBADF: Bad file number

Stack backtrace:
   0: <E as anyhow::context::ext::StdError>::ext_context
   1: <librqbit::storage::filesystem::fs::FilesystemStorage as librqbit::storage::TorrentStorage>::pwrite_all_vectored
   2: librqbit::torrent_state::live::PeerHandler::on_received_piece::write_to_disk
   3: tokio::runtime::scheduler::multi_thread::worker::block_in_place
   4: <librqbit::torrent_state::live::PeerHandler as librqbit::peer_connection::PeerConnectionHandler>::on_received_message
...