fpta: subtree-pull libmdbx upstream (zero mode_t as open-existing flag).
fpta: subtree-pull libmdbx upstream (zero mode_t as open-existing flag).

Merge commit '14ae9fb2a1101bc8eb1b06967d7431a4b715aa1e' into devel

leo-yuriev/libfpta,leo-yuriev/libfpta