- Rename `syscall_linux.go` to `syscall.go` with build constraints for non-BSD and non-Windows platforms.
- Merge `syscall_darwin.go` into `syscall_bsd.go` and adjust build constraints for BSD platforms (Darwin, FreeBSD, OpenBSD).
- Remove redundant `syscall_freebsd.go`.
- Add build constraints to `syscall_windows.go` for Windows platform.