Merge app_ioctl into zfs_ioctl and remove printfs.
Recognize and use an installed spl module.
Implement fstatat64() and openat64() for OS X.

This fixes probing directories for vdevs, used by zpool import -d.
The implementations are borrowed (and improved on) from OpenBSM, and
essentially perform a pushd/popd with {fstat,open}() inside.
Remove debug printf and unused variable.