Add conditional chkconfig to packaging

Prior to adopting the kmod style packaging the zfs packages
would conditionally invoke /sbin/chkconfig to create the
proper links for the init script.  This is done conditionally
because many distributions are moving away from SysV style
init scripts and we don't want to cause errors on those.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1376

rpm/generic/zfs.spec.in

@@ -113,7 +113,14 @@ make %{?_smp_mflags}
 make install DESTDIR=%{?buildroot}
 find %{?buildroot}%{_libdir} -name '*.la' -exec rm -f {} \;
 
-%post -p /sbin/ldconfig
+%post
+/sbin/ldconfig
+[ -x /sbin/chkconfig ] && /sbin/chkconfig --add zfs
+
+%preun
+if [ $1 -eq 0 ] ; then
+    [ -x /sbin/chkconfig ] && /sbin/chkconfig --del zfs
+fi
 
 %postun -p /sbin/ldconfig