Raise safe max MDB_MAXKEYSIZE.

Use a sub-DB for DUPSORT item #1/#2 per key if needed: Not a sub-
page too big for a node, nor an overflow page (which not all DUPSORT
code checks for). Move "insert" code, to avoid non-loop goto upwards.

(This is the commit which needs the change to xdata.mv_size in
commit 9d6e4a91 "page sizes".)