Extract mknod, umask, lstat to pkg/system

Some parts of pkg/archive is called on both client/daemon code. To get
it compiling on Windows, these funcs are extracted into files with
build tags.

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

system/lstat.go

@@ -0,0 +1,16 @@
+// +build !windows
+
+package system
+
+import (
+	"syscall"
+)
+
+func Lstat(path string) (*syscall.Stat_t, error) {
+	s := &syscall.Stat_t{}
+	err := syscall.Lstat(path, s)
+	if err != nil {
+		return nil, err
+	}
+	return s, nil
+}