Remove zfs service only on uninstall, not on upgrade

Remove zfs service only on uninstall, not on upgrade

This caused problems on upgrade using RPM:

* The new version will run chkconfig --add, which has no effect
  since the service was already added.

* The old version will run chkconfig --del, which caused zfs
  service removal.

Only run "chkconfig --del" on complete uninstall, by checking
the value of "$1" to %preun, which will be "0" on uninstall,
and "1" on upgrade.

  http://www.rpm.org/max-rpm/s1-rpm-inside-scripts.html

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #314