mixing single and multi-entrance caves.
I also added the alternate door frame table, which is needed for having
single entrance caves at
Sanctuary and Hyrule castle. (Added as a table rather than hard coding
the ids in order to be more friendly to level editors).
Updated the comments and function name to be clear that we are tracking
the overworld door
id, not the entrance id.
Remove the code that attempts to clear the entrance because the code is
broken (it lacked a needed ".w" suffix, so xkas miscompiled it) meaning
the value was never cleared, which has not caused any issues. Also
having the entrance ID around is generally useful.
