Add systemd unit files for ZFS startup

[Lalufu]

This adds systemd unit files replacing the functionality offered by
the SysV init script found in etc/init.d.

It has been developed and tested on Fedora 19, Fedora 20
and openSuSE 13.1.

Four unit files and one target are offered.

zfs-import-cache.service:
Import pools from /etc/zfs/zpool.cache. This unit will wait for
udev to settle.
zfs-import-scan.service:
Import pools by scanning /dev/disk/by-id for zvols. This unit will
only run if /etc/zfs/zpool.cache is not present. This unit will wait
for udev to settle
zfs-mount.service:
Mount ZFS native filesystems. It contains a dependency to be loaded
before local-fs.target.
zfs-share.service:
Share NFS/SMB filesystems. This unit contains a dependency that
will cause it to be restarted whenever the smb or nfs-server unit
is restarted, restoring the shares added.
zfs.target:
This target pulls in the other units in order to start ZFS. It's
the only unit that can be enabled/disabled, all other services
are static and pulled in by dependencies. It will honour zfs=off
and zfs=no options on the kernel command line.

[behlendorf]

@Lalufu Thank you for taking care of this, I've just merged it to master.

[dweeezil]

FWIW, I finally got a chance to try this on a F19 system and my pool was imported but none of the filesystems were mounted. A systemctl start zfs-mount did mount them. I did run systemctl status zfs-mount before doing the "start" but I can't remember what it output. I've not yet had a chance to look into it any further.

[behlendorf]

I saw similar issues on FC19 which were caused by selinux. They still need to be investigated.