clone fails on FreeBSD on ZFS. Bug report thru FreeBSD.

Make sanakirja portable.

./sanakirja/src/environment/mod.rs:            fallocate(file, offset + length)?;
./sanakirja/src/environment/mod.rs:fn fallocate(file: &std::fs::File, length: u64) -> Result<(), Error> {
./sanakirja/src/environment/mod.rs:fn fallocate(&self, length: u64) -> Result<(), Error> {

It’s worth adding, this reference #857