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_unsupported.go

@@ -1,13 +1,16 @@
-// +build !linux
+// +build !linux,!windows
 
 package system
 
-import "syscall"
+import (
+	"syscall"
+)
 
-func GetLastAccess(stat *syscall.Stat_t) syscall.Timespec {
-	return stat.Atimespec
-}
-
-func GetLastModification(stat *syscall.Stat_t) syscall.Timespec {
-	return stat.Mtimespec
+func fromStatT(s *syscall.Stat_t) (*Stat, error) {
+	return &Stat{size: s.Size,
+		mode: uint32(s.Mode),
+		uid:  s.Uid,
+		gid:  s.Gid,
+		rdev: uint64(s.Rdev),
+		mtim: s.Mtimespec}, nil
 }