IPRT: Added 'off' parameter to RTVfsIoStrmSgWrite and RTVfsIoStrmSgRead so it's easier to write passthru layers. Added RTVfsIoStrmReadAll, RTVfsIoStrmReadAllFree, RTVfsIoStrmFromBuffer, RTManifestPtIosIsInstanceOf, RTCrX509Certificate_ReadFromBuffer and RTCrDigestUpdateFromVfsFile. Updated the manifest passthru read code to handle ReadAt requests which skips parts and jumps back to re-read stuff on streams/files which are seekable.
