Revert the native impl of kvvfs's xOpen() (as distinct from the JS impl which the...

Revert the native impl of kvvfs's xOpen() (as distinct from the JS impl which the wasm build uses) to the historical db name restrictions of 'local' or 'session', failing with SQLITE_CANTOPEN if passed another name. [ec866b04d088e53b] overhauled support for kvvfs db names in JS but it turns out that the native impl still relies on those names in order to match journals to databases. Correct a related too-lenient assert() in the WASM pieces.

FossilOrigin-Name: 4b3ec30c63e3824163a2e6cacceb9630301b61f3ffd2b80f8a7b234f09bc251b