Fix integer data types in BTEs

[thomasm]

For instance, the os block size in source:/trunk/tpie/bte/stream_ufs.h should probably be unsigned, but since TPIE_OS_WRITE has no other way of returning error than returning -1, everything needs to be signed.. But this breaks source:/trunk/tpie/bte/stream_ufs.h#L524 since the return type of "sizeof" is unsigned and this forces us to static_cast our way out.