Disregard superuserness when checking to see if a role GRANT would

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.