Don't allow new pages for free list to come from the free list.
Otherwise a nasty data self-reference occurs that is too much
trouble to unwind.

Can re-use free pages 1 txn earlier
Must update all dbs tables on txn commit

Also in cursor_set, check the current page (if already set from
before) before starting over again from the root.

For better space utilization

If O_DSYNC is not defined, that may be due to poor compiler options.
We could fall back to the less efficient O_SYNC, but it seems better
to let the user learn of the problem and give better compiler options.

C99 says DPRINTF(fmt, ...) must get at least 2 args.  Using DPUTS for 1 arg.

Also make sure dups are counted in md_entries

Write the first datum normally. Convert to a sub-db if
additional items for the same key are written.

Replace MDB_FAIL with more specific codes
Return MDB_PANIC after failure to write meta page