Distinguish between MaxHeapAttributeNumber and MaxTupleAttributeNumber,

Distinguish between MaxHeapAttributeNumber and MaxTupleAttributeNumber,
where the latter is made slightly larger to allow for in-memory tuples
containing resjunk attributes.  Responds to today's complaint that one
cannot UPDATE a table containing the allegedly-legal maximum number of
columns.

Also, apply Manfred Koizar's recent patch to avoid extra alignment padding
when there is a null bitmap.  This saves bytes in some cases while not
creating any backward-compatibility problem AFAICS.