fs/AllocatedPath: API documentation grammar fixes
fs/AllocatedPath: add method Steal()
storage/nfs: move code to UriToNfsPath()
storage/nfs: use string::append() instead of string::insert()

Swap the order of adding the URI and the slash, because appending is
cheaper than inserting.
storage/nfs: convert file name to UTF-8

Assume the configured filesystem character set is also used by the NFS
server.