Fix cursor_del0 cursor fixup

Skip cursors that aren't as deep as current cursor
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.
Change default mem init behavior

Init malloc'd memory by default, use #MDB_NOMEMINIT to disable.
Bump version to 0.9.10
Clarify WRITEMAP / NOMEMINIT
Merge remote-tracking branch 'origin/mdb.master' into OPENLDAP_REL_ENG_2_4
Windows/Win64/MSVC tweaks

Silence warnings, fix broken MinGW64 definitions, etc.
Bump to 0.9.10
Doc tweak

Measured perf cost of meminit was 7% worst case on an Intel Core2
system. YMMV.
ITS#7743 Fix indexed AND filter

Partially revert 47f6aec61e65afd66bea096887f1cd550b55946e
Was using RANGE_FIRST/RANGE_LAST on an IDL known to *not* be a range.
ITS#7743 bits
Merge remote-tracking branch 'origin/mdb.master' into OPENLDAP_REL_ENG_2_4
ITS#7746 fix for cert with NULL issuerDN
ITS#7746
Prep for release
ITS#7749 propagate op->o_extra in syncprov_findbase
ITS#7749