Fix darwin issues

[thehajime]

This PR is related to containerd/containerd#4526, which tries to support darwin runtime of containerd.

The patches contains several fixes:

• a fix to incorrect option for du command

% TMPDIR=/tmp/fs go test ./fs -v -run TestUsage
(snip)
=== CONT  TestUsage/HardlinkSparefile
    du_test.go:134: Failed calling du: exit status 64
du: illegal option -- -
usage: du [-H | -L | -P] [-a | -s | -d depth] [-c] [-h | -k | -m | -g] [-x] [-I mask] [file ...]
=== CONT  TestUsage/Hardlinks
    du_test.go:134: Failed calling du: exit status 64
(snip)

• utimes usage similar to FreeBSD (bb87682)

As macOS is similar to FreeBSD, the fixes are actually following the way FreeBSD did, but darwin itself has slightly different internals, thus adapting those caes.

Even du command fixes, the test with TestUsage still fails with SparseFiles, but will address this issue later.

[estesp]

LGTM