matroska: redo seekhead handling

In particular, this reads chained seekheads. This makes seeking faster
in files which have the index indirectly linked through 2 seekheads.

As a side-effect, this warns when reading level-1 (toplevel) elements
multiple times (other than seekheads, clusters, and void/crc). Such
elements are not valid and likely break everything.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
