Tweak mdb_mid2l_insert() call from ITS#7775.

It does not return MDB error codes.
Update MDB_PANIC description
Revert "Update MDB_PANIC description" (thinko).

This reverts commit 6bd34d0534f1c8bfd73deaf70febd3b58a2f9d5a.
Revert "ITS#7793 mdb_cursor_put(): Fix MDB_CURRENT."

This reverts commit 4e954f8ae83d5b39736bab2da0eaf2bdf5ee3961.
Caused ITS#7828.
ITS#7828, #7793

Clarify MDB_CURRENT key usage, simplify freelist_save
More for ITS#7793/7828

Note MDB_CURRENT/MDB_DUPSORT requirement
ITS#7793/7828

Fix md_entries count, test for insert on md_entries increment
ITS#7829 fix rebalance

Fixup cursor state after node_move/page_merge
ITS#7829 more for prev commit
ITS#7829 Simplify last commit
ITS#7829 more for mdb_rebalance

Make sure to propagate rebalance results all the
way back to original caller's cursor.
Fix mdb_rebalance collapsing root

Shift the rest of the cursor stack as needed
More ITS#7793 Check key->mv_size with MDB_CURRENT.

Since non-NULL key must be provided now.
Document DBI issues, extend MDB_BAD_VALSIZE.
ITS#7771 mdb_cursor_put doc: Cursor moves on error
ITS#7861 Re-fix MDB_db.md_entries.

mdb_cursor_del(): Fix md_entries with MDB_NODUPDATA.
mdb_cursor_put(): Check in xcursor whether an item got inserted. Count
data instead of keys after 13d6d59c512065db3741c20c65b4d9928b986968.
ITS#7834 Fix MDB_MULTIPLE writing multiple keys.

Update the state before looping, so the key is not duplicated.
Broke in 4c8f57615c5ca7b014c038e59c1045182e74f5ad (mostly).
Simplify MDB_MULTIPLE
Factor out MDB_TXN_ERROR
More for big MDB_MAXKEYSIZE with MDB_DUPSORT.

With mdb_cursor_put(new key, data item big enough to need a sub-DB):
- Track cursors as when adding other nodes.  The "!do_sub" is now
  unneeded.  Before ITS#7861 it distinguished inserting key from data.
- Set MDB_TXN_ERROR if an empty sub-DB was created.  I.e. adding
  key:subDB succeeded, but inserting the data item failed.