zfsutils-linux: persist hostid in postinst script

/etc/hostid does not exist on a minimal debian system. ZFS reads its contents for MMP (zpool property multihost set to on), and needs the value to be stable.

Before the SPL->ZFS merge the spl userspace package wrote the file (by parsing/ mangling the output of hostid (1)) Since ZFS provides the zgenhostid script, which creates a random 4 byte hostid and writes it to /etc/hostid, we use that instead. (hostid (1) default to mangling a configured ip of the system, which can be the same across multiple installs (install happens with the same ip)

Signed-off-by: Stoiko Ivanov s.ivanov@proxmox.com