Use sparse files (FALLOC_FL_PUNCH_HOLE) to deallocate unused pages on disk

The README currently says:

https://github.com/etcd-io/bbolt/blob/dbe1e6dc0d30fc1ae4c7c8078615dd2bbccf4499/README.md#L886-L891

The missing ability to truncate to return space to the filesystem could be mitigated, at least on ext4/xfs/btrfs on linux, by having bbolt call

```
unix.Fallocate(fd, unix.FALLOC_FL_PUNCH_HOLE|unix.FALLOC_FL_KEEP_SIZE, offset, length)
```

either eagerly whenever a page is added on the freelist or lazily later during a background operation, with `offset` the offset of the page in bytes from the start of the file, and `length` the size of the page in bytes.

This would instruct the kernel/fs to free the space on disk corresponding to that page, thereby making it available again to the rest of the system.tio

Given that the actual effectiveness of this approach depends on the combination of (at least) OS, OS version, filesystem, filesystem page size, database page size, and physical disk page size, whether to perform the Fallocate call should be controlled by a database option, defaulting to false.

Note: this is the same as https://github.com/etcd-io/bbolt/issues/125, that was automatically closed as stale due to no activity.