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

Log Message:
-----------
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.

Modified Files:
--------------
    pgsql/contrib/intagg:
        int_aggregate.c (r1.15 -> r1.16)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/intagg/int_aggregate.c.diff?r1=1.15&r2=1.16)
        int_aggregate.sql.in (r1.7 -> r1.8)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/intagg/int_aggregate.sql.in.diff?r1=1.7&r2=1.8)
    pgsql/doc/src/sgml:
        xaggr.sgml (r1.26 -> r1.27)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/xaggr.sgml.diff?r1=1.26&r2=1.27)
        xfunc.sgml (r1.99 -> r1.100)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/xfunc.sgml.diff?r1=1.99&r2=1.100)
    pgsql/src/backend/executor:
        nodeAgg.c (r1.128 -> r1.129)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeAgg.c.diff?r1=1.128&r2=1.129)
    pgsql/src/backend/utils/adt:
        int8.c (r1.57 -> r1.58)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/int8.c.diff?r1=1.57&r2=1.58)