Handle errors. Fix cond_wait condition so mc_new
is the sole control var. Drop specious cond_waits.
Do not look at 'mo' while copythr writes it.
Preserve DB flags (use metapage#1) when main DB is empty.
Fail if metapage root != actual root in output file.
Some _aligned_malloc() doc seems to think arg NULL = user error.
Don't know if posix_memalign() pointer is defined after failure.
Change-Id: Idfdc118b4848bb96bace0f29db9dcdd710b7a1f4
