Fallback path for mmap

[database64128]

If the db is on a file system without mmap support, like a remote share or some FUSE filesystem, mmap(2)/MapViewOfFile may fail. Now that we target Go 1.23+, there's an easy way to detect this with errors.Is(err, errors.ErrUnsupported) (added in Go 1.21). This seems to work quite well in my project: https://github.com/database64128/shadowsocks-go/blob/f299d1bb61ba85840dbfbf948adeb8d2b4c216c4/mmap/mmap.go#L39

If you are OK with this idea, I can open a PR to implement it.

[oschwald]

Sure, I'd be happy to accept a PR to do that. Please mention it in the documentation for the function as well.