pgsql: Disallow aggregate functions in UPDATE commands (unless within a

Log Message:
-----------
Disallow aggregate functions in UPDATE commands (unless within a sub-SELECT).
This is disallowed by the SQL spec because it doesn't have any very sensible
interpretation.  Historically Postgres has allowed it but behaved strangely.
As of PG 8.1 a server crash is possible if the MIN/MAX index optimization gets
applied; rather than try to "fix" that, it seems best to just enforce the
spec restriction.  Per report from Josh Drake and Alvaro Herrera.

Modified Files:
--------------
    pgsql/src/backend/parser:
        analyze.c (r1.334 -> r1.335)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/analyze.c.diff?r1=1.334&r2=1.335)