pgsql: Minor correction: cause ALTER ROLE role ROLE rolenames to behave

Log Message:
-----------
Minor correction: cause ALTER ROLE role ROLE rolenames to behave
sensibly, even though we don't document it.

Modified Files:
--------------
pgsql/src/backend/commands:
user.c (r1.158 -> r1.159)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/user.c.diff?r1=1.158&r2=1.159)
pgsql/src/backend/parser:
gram.y (r2.503 -> r2.504)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/gram.y.diff?r1=2.503&r2=2.504)