bundle2: make header reading optional The `readbundle` function will consume the 4 first bytes to dispatch between various unbundler. We introduce a way to inform `unbundle20` that the header has been read and it can be trusted.

bundle2: prepare readbundle to return more that one type of bundle We first read 4 bytes to get the `HG10` bytes then we read the compression scheme if this is `HG10`. This prepares the code for the arrival of `HG20` handling.

bundle2: add a ui argument to readbundle The bundle2 unbundler needs a ui argument. We are now passing this information to `readbundle`.