pgsql-server: Guard against transaction control statements in SQL

Log Message:
-----------
Guard against transaction control statements in SQL functions. This
never worked, but it particularly doesn't work now.

Modified Files:
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pgsql-server/src/backend/executor:
functions.c (r1.86 -> r1.87)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/executor/functions.c.diff?r1=1.86&r2=1.87)