Row Level Security GUC and Role Attribute.

added initial RLS enabled GUC.
 - supports SET ROW SECURITY { ON | OFF }, as well as standard GUC
   set/reset commands.
 - global setting to disable row security.
 - can be set by any user.
added ALTER ROLE <role> { BYPASSRLS | NOBYPASSRLS } functionality.
 - BYPASSRLS - role is allowed to bypass row level security.
 - NOBYPASSRLS - role is not allowed to bypass row level security.
added --allow-row-security option to pg_dump.
 - allows user to enable row security.

Notes:
 - does not include proper plancache invalidation.