Add flags to ensure memory mapping (or preload into memory) and to ensure seeking

[vampirefrog]

Add STREAM_ENSURE_MEMORY_ACCESS flag to ensure that memory access will be available. For gzipped streams or for zip file streams, load into memory.

Add STREAM_ENSURE_SEEK_SET, STREAM_ENSURE_SEEK_START, STREAM_ENSURE_SEEK_END to ensure that these types of seek are available. For example, gzip does not offer SEEK_END.

Add STREAM_ENSURE_FAST_SEEK to load gzip files into memory because forward seeking is slow.