symm/seal.c: Spruce up a bit.

symm/seal.c: Spruce up a bit.

I'm not deploying the reservoir code here because the core update is
entangled with the buffering in an unusual way, to avoid having to spill
the working state.  Switching to the reservoir logic would mean having
to factor out the core update, which would lead to spillage.

Instead, settle for renaming the buffering variables (and switching
which end we count from) and reformatting the code a bit.