pgsql: Give an explicit error for serial[], rather than silently

Log Message:
-----------
Give an explicit error for serial[], rather than silently ignoring
the array decoration as the code had been doing.

Modified Files:
--------------
pgsql/src/backend/parser:
parse_utilcmd.c (r2.9 -> r2.10)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_utilcmd.c?r1=2.9&r2=2.10)