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.go

@@ -0,0 +1,42 @@
+package system
+
+import (
+	"syscall"
+)
+
+type Stat struct {
+	mode uint32
+	uid  uint32
+	gid  uint32
+	rdev uint64
+	size int64
+	mtim syscall.Timespec
+}
+
+func (s Stat) Mode() uint32 {
+	return s.mode
+}
+
+func (s Stat) Uid() uint32 {
+	return s.uid
+}
+
+func (s Stat) Gid() uint32 {
+	return s.gid
+}
+
+func (s Stat) Rdev() uint64 {
+	return s.rdev
+}
+
+func (s Stat) Size() int64 {
+	return s.size
+}
+
+func (s Stat) Mtim() syscall.Timespec {
+	return s.mtim
+}
+
+func (s Stat) GetLastModification() syscall.Timespec {
+	return s.Mtim()
+}