this is an aclmask function and does not have the same return convention
as aclcheck functions.  Also adjust the behavior so that users without
CREATE TEMP permission still have USAGE permission on their session's
temp schema.  This allows privileged code to create a temp table and
make it accessible to code that's not got the same privilege.  (Since
the default permissions on a table are no-access, an explicit grant on
the table will still be needed; but I see no reason that the temp schema
itself should prohibit such access.)