Fix sys.stdin.seek() problem properly.

The last hotfix to make sys.stdin seek()-able was far from being
proper. It checked only for sys.stdin (which is only one possible
stream delivering unseekable file positions) and didn't consider that
sys.stdin could be replaced (by somthing with proper seek
functionality).

We now test properly for working seek() functionality. If the stream
does not support it, we copy streamdata to a new temporary file. If
the file is not too large, we keep the file data in memory (up to 20
MB), so in most cases runtime trade-off should be moderate.