Fix prev commit, cursor depth comparison
mdb_cursor_put(): Factor out code. Kill a label.
ITS#7713 Handle bigger OS page sizes

Use DB page size = min(32k, OS pagesize).  Previous limit was 8k
(MDB_MINKEYS*MDB_PAGESIZE).

Handle DB pagesize < OS pagesize. That's an I/O pessimization,
but transactions remain atomic: Only writing the MDB_meta must
be atomic, and it fits in one OS page.

Don't truncate desired subpage size: Asssign it to a size_t
(mv_size), not an uint16_t (mp_upper).
Fix MDB_MAXKEYSIZE doc: Account for MDB_DUPSORT.

A node may need room for [key, subpage with 2 items] = 3 keys.
Add MDB_CLEANMEM option.

Initialize unused portions of DB pages, for security or to silence
checkers like Purify and Valgrind. Like BDB configure --enable-umrw.