Refactor pkg/archive with a platform-independent stat struct

pkg/archive contains code both invoked from cli (cross platform) and
daemon (linux only) and Unix-specific dependencies break compilation on
Windows. We extracted those stat-related funcs into platform specific
implementations at pkg/system and added unit tests.

Signed-off-by: Ahmet Alp Balkan <ahmetb@microsoft.com>

system/stat_test.go

@@ -0,0 +1,34 @@
+package system
+
+import (
+	"syscall"
+	"testing"
+)
+
+func TestFromStatT(t *testing.T) {
+	file, _, _ := prepareFiles(t)
+
+	stat := &syscall.Stat_t{}
+	err := syscall.Lstat(file, stat)
+
+	s, err := fromStatT(stat)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if stat.Mode != s.Mode() {
+		t.Fatal("got invalid mode")
+	}
+	if stat.Uid != s.Uid() {
+		t.Fatal("got invalid uid")
+	}
+	if stat.Gid != s.Gid() {
+		t.Fatal("got invalid gid")
+	}
+	if stat.Rdev != s.Rdev() {
+		t.Fatal("got invalid rdev")
+	}
+	if stat.Mtim != s.Mtim() {
+		t.Fatal("got invalid mtim")
+	}
+}