ITS#7913 mdb_cursor_get(*_DUP) cleanup

FIRST_DUP/LAST_DUP should succeed even on non-duplicate values
PREV_DUP should reset cursor's EOF flag.
More for MIPS

Paranoia for 3630066843b7ca6b2cd12911d3e2fe3314cd4549 do the
cacheflush before setting mti_txnid.
ITS#7793 update branch key if needed
ITS#7793 doc update, again
ITS#7793 more for branch key update
More for MIPS

Do a cacheflush after writing data pages, separate from meta's cacheflush
Just use memalign, not posix_memalign

Everything has it. Solaris doesn't have posix_memalign.