Fix the way we handle /etc/hostid

Fix the way we handle /etc/hostid

  * /etc/hostid should not be a configuration file.
    This is by policy:
    E.2 Fully-featured maintainer script configuration handling
    http://www.debian.org/doc/debian-policy/ap-pkg-conffiles.html

    This kind of site-specific configuration files should not be added
    to the package files.
    You don't want this file to be removed when you purge the package.

  * Handle the creation on /etc/hostid on zfs-dkms.postinst

    * Check if /etc/hostid exists and is at least 4 bytes size.

      * If it isn't, then write the value returned by "hostid" on /etc/hostid
        taking care of endianness

    * Note that on zfs-dkms.postinst #DEBHELPER# is after this check for
      /etc/hostid. Therefore the stabilization of /etc/hostid will be
      done before the spl mode is built by DKMS.