Rename 'zed.service' to 'zfs-zed.service'

For consistency all systemd unit files and init scripts now share
the same names. This prevents an issue where the zed is started
twice on systems where both the systemd and sysv infrastructure is
installed concurrently.

For backward compatibility a 'zed' alias has been added. This
allows the user to interact with the service using either the
name 'zed' or 'zfs-zed'.

Signed-off-by: Turbo Fredriksson <turbo@bayour.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #3837