Refactor mark chains.

* Rename MD_MARKCHAIN to MD_MARKSTACK to indicate its semantics much
  clearer.
* Simplify its implementation (single-linked list instead of
  double-linked one).
* Where it was reused (misused?) for other, unrelated stuff, with other
  semantics, it's now done explicitly. (i.e. got rid of
  TABLECELLBOUNDARIES).
* PTR_CHAIN still uses the stack (we don't care about order there), but
  it got separated from the array of ordinary opener stacks at least.