Add pg_has_role() family of privilege inquiry functions modeled after the

Add pg_has_role() family of privilege inquiry functions modeled after the
existing ones for object privileges.  Update the information_schema for
roles --- pg_has_role() makes this a whole lot easier, removing the need
for most of the explicit joins with pg_user.  The views should be a tad
faster now, too.  Stephen Frost and Tom Lane.