Move the secpolicy_sys_config implementation to #ifdef __OPPLE__

The immediate reason for providing an implementation of the
secpolicy_sys_config privilge check was to work around the bug that
was causing scrub to hang when run without root privilges. Now that
lundman has found an actual fix for that bug, I have disabled the
secpolicy_sys_config implementation and restored

https://github.com/openzfsonosx/zfs/issues/216