Adjust the API for aggregate function calls so that a C-coded function

Adjust the API for aggregate function calls so that a C-coded function
can tell whether it is being used as an aggregate or not.  This allows
such a function to avoid re-pallocing a pass-by-reference transition
value; normally it would be unsafe for a function to scribble on an input,
but in the aggregate case it's safe to reuse the old transition value.
Make int8inc() do this.  This gets a useful improvement in the speed of
COUNT(*), at least on narrow tables (it seems to be swamped by I/O when
the table rows are wide).  Per a discussion in early December with
Neil Conway.  I also fixed int_aggregate.c to check this, thereby
turning it into something approaching a supportable technique instead
of being a crude hack.