X wcfs: try to set sticky for /tmp/wcfs even if the directory already exists

This should cure the situation when on testnodes /tmp/wcfs already exists
created without sticky bit.

wcfs/__init__.py

@@ -295,10 +295,20 @@ def _wcfs_exe():
 def _mntpt_4zurl(zurl):
     m = hashlib.sha1()
     m.update(zurl)
+
     # mkdir /tmp/wcfs with stiky bit. This way multiple users can create subdirectories inside.
     wcfsroot = "%s/wcfs" % (tempfile.gettempdir())
+    wcfsmode = 0777 | stat.S_ISVTX
     if _mkdir_p(wcfsroot):
-        os.chmod(wcfsroot, 0777 | stat.S_ISVTX)
+        os.chmod(wcfsroot, wcfsmode)
+    else:
+        # migration workaround for the situation when /tmp/wcfs was created by
+        # code that did not yet set sticky bit.
+        _ = os.stat(wcfsroot)
+        if _.st_uid == os.getuid():
+            if _.st_mode != wcfsmode:
+                os.chmod(wcfsroot, wcfsmode)
+
     mntpt = "%s/%s" % (wcfsroot, m.hexdigest())
     _mkdir_p(mntpt)
     return mntpt