Use native df(1) in the ZFS test suite

Eliminate the weird Illumos partial-compatibility wrapper.

tests/sys/cddl/zfs/bin/Makefile
tests/sys/cddl/zfs/bin/df.ksh

Delete the compatibility wrapper

tests/sys/cddl/zfs/include/libtest.kshlib
tests/sys/cddl/zfs/tests/clean_mirror/cleanup.ksh
tests/sys/cddl/zfs/tests/cli_root/zfs_upgrade/zfs_upgrade.kshlib
tests/sys/cddl/zfs/tests/cli_root/zfs_upgrade/zfs_upgrade_005_pos.ksh
tests/sys/cddl/zfs/tests/devices/devices_common.kshlib
tests/sys/cddl/zfs/tests/largest_pool/largest_pool_001_pos.ksh
tests/sys/cddl/zfs/tests/scrub_mirror/cleanup.ksh

Adjust usage of df

Sponsored by: Spectra Logic Corp