By calling devfs_make_node_clone we can specify minor_alloc callback
and patch that into zfsdev_minor_alloc(), causing a unique minor to be
allocated for each open. We then need to create softstate for ctldev so
we can differentiate between ctldev and zvol ioctls.
Issue #173
zfs_onexit needed to map fd to minor (with spl help) to lookup the
soft node.
Fix zevents' onexit as well, clean up old #ifdef APPLEs.
Remove debug prints