Change startup mode of ZED

Change the startup mode of ZED to non-forking. While systemd can
track processes that detach from the terminal just fine, running
processes in non-forking mode is the preferred mode of operation.

Also remove user/group definitions as root/root is the default.

Signed-off-by: Chris Dunlap <cdunlap@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2252

etc/systemd/system/zed.service.in

@@ -5,9 +5,5 @@ After=zfs-import-cache.service
 After=zfs-import-scan.service
 
 [Service]
-Type=forking
-ExecStart=@sbindir@/zed
-PIDFile=@runstatedir@/zed.pid
-User=root
-Group=root
+ExecStart=@sbindir@/zed -F
 Restart=on-abort