Do not `mmap(2)` disk devices on FreeBSD

[danfe]

Currently, class memory_map which provides mmap(2)-based access for a device and the code using it are guarded with #ifndef _WIN32. These bits should be excluded on FreeBSD as well (see this discussion which gives some rationale as to why).

[maharmstone]

Thanks for this - fixed by https://github.com/maharmstone/ntfs2btrfs/commit/1e0115b7faf4b72a1c5757165874645cf03663cc. Thanks also for adding it to Ports.