Support FreeBSD on pkg/system/utimes_*.go

Implement system.LUtimesNano and system.UtimesNano. The latter might be
removed in future because it's basically same as os.Chtimes. That's why
the test is mainly focusing LUtimesNano.

Docker-DCO-1.1-Signed-off-by: Kato Kazuyoshi <kato.kazuyoshi@gmail.com> (github: kzys)

system/utimes_freebsd.go

@@ -0,0 +1,24 @@
+package system
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+func LUtimesNano(path string, ts []syscall.Timespec) error {
+	var _path *byte
+	_path, err := syscall.BytePtrFromString(path)
+	if err != nil {
+		return err
+	}
+
+	if _, _, err := syscall.Syscall(syscall.SYS_LUTIMES, uintptr(unsafe.Pointer(_path)), uintptr(unsafe.Pointer(&ts[0])), 0); err != 0 && err != syscall.ENOSYS {
+		return err
+	}
+
+	return nil
+}
+
+func UtimesNano(path string, ts []syscall.Timespec) error {
+	return syscall.UtimesNano(path, ts)
+}