Merge duplicate MD code into MI sr_open_chunk()

Merge duplicate MD code into MI sr_open_chunk()

It does not have the prettiest signature, but nicely folds identical copies
into softraid.c, which then allows us to reuse sr_open_chunk() yet again in
an upcoming diff to make -p softraid aware (fixes arm64 installations).

Regress keeps passing.
Works fine on amd64, arm64 and sparc64.
"looks fine" mlarkin for whom this unbreaks a fresh arm64 softraid install