MDMF: remove extension fields from caps, tolerate arbitrary ones. Fixes #1526

MDMF: remove extension fields from caps, tolerate arbitrary ones. Fixes #1526

The filecaps used to be produced with hints for 'k' and segsize, but they
weren't actually used, and doing so had the potential to limit how we change
those filecaps in the future. Also the parsing code had some problems dealing
with other numbers of extensions. Removing the existing fields and making the
parser tolerate (and ignore) extra ones makes MDMF more future-proof.