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