Re-enable zfs-dracut packaging

This was disabled with f044301bacbe69848fd4c0fcd02f5b4d08d7f298 in January 2012 as it was broken back then, but the situation has improved vastly in the meantime. The zfs-dracut package not only works well with dracut-038 on Debian Jessie but offers one significant advantage over initramfs-tools: The ability to export a ZFS root on shutdown, eliminating the necessity to force-import on every reboot. A patch to achieve that was submitted yesterday with zfsonlinux/zfs#2766. (I can add that patch to this pull request if you want.) Note that there's currently a bug in Debian's dracut_038-2 package which prevents the systemd unit files from being included in the package. I'll submit a bug report to the maintainer in a bit and will add the bug number in a comment here. Without that fix, exporting the ZFS root on shutdown doesn't work.

I've also added a dependency to package zfs-initramfs on initramfs-tools, analogously to the dependency of package zfs-dracut on dracut. This ensures that a user may install only one of zfs-dracut and zfs-initramfs at the same time since dracut and initramfs-tools conflict.

I'm not sure if this is the correct branch to submit this pull request, all others seemed stale. Please let me know if I should re-submit to a different branch. Thanks.