pgsql: Disregard superuserness when checking to see if a role GRANT

Log Message:
-----------
Disregard superuserness when checking to see if a role GRANT would
create circularity of role memberships. This is a minimum-impact fix
for the problem reported by Florian Pflug. I thought about removing
the superuser_arg test from is_member_of_role() altogether, as it seems
redundant for many of the callers --- but not all, and it's way too late
in the 8.1 cycle to be making large changes. Perhaps reconsider this
later.

Modified Files:
--------------
pgsql/src/backend/commands:
user.c (r1.163 -> r1.164)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/user.c.diff?r1=1.163&r2=1.164)
pgsql/src/backend/utils/adt:
acl.c (r1.126 -> r1.127)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/acl.c.diff?r1=1.126&r2=1.127)
pgsql/src/include/utils:
acl.h (r1.85 -> r1.86)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/acl.h.diff?r1=1.85&r2=1.86)