mfs: use native path even for mfs.

Using only posix would work only if the only code doing path resolving was MFS, but store has several path.resolves which on windows breaks the compactibility and instead of directories we get files with path names similar to "c:\store\.." instead of it being separate directories.

lib/mfs.js

@@ -7,9 +7,8 @@
 'use strict';
 
 const assert = require('bsert');
-const path = require('path');
 const os = require('os');
-const Path = path.posix || path;
+const Path = require('path');
 
 /*
  * Constants