support read-only downloaded files

[milahu]

currently rqbit throws when the downloaded files are read-only

finished with error: error opening "/path/to/file.txt" in read/write mode

the downloaded files are complete and rqbit should only seed them

$ chmod -R -w ~/downloads

$ rqbit server start ~/downloads
ERROR torrent{id=0}:initialize_and_start: librqbit_core::spawn_utils: finished with error: error opening "/path/to/file.txt" in read/write mode