Add systemd unit file for zed

This commit adds a systemd unit file for zed.service and integrates
it into the zfs.target from commit 881f45c.

Signed-off-by: Chris Dunlap <cdunlap@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #2108
Issue #2