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

Use sync writes in mtest.