`fcntl` module does not exist on Windows

`warcprox/writer.py` imports `fcntl` to acquire an exclusive lock on files opened for writing, but this fails on Windows because the `fcntl` module is only available on Unix-like systems. It should either gracefully degrade to not use file locking if the module is not available, or use a portable library like [`portalocker`](https://pypi.org/project/portalocker/).

Start in warcprox/writer.py at the fcntl import and the file-locking path used for files opened for writing. Check how the module behaves when fcntl is unavailable and compare graceful degradation with using portalocker. Done means the module can run on Windows without failing at the import or locking step, with the chosen behavior verified there.

Written by the indexing model from the issue text.