RLS - update pg_dump and pg_restore for row-security.
RLS - update row-security grammar

* Removed command from DROP POLICY.
* Added optional command and roles for ALTER POLICY.
RLS - resolve comment typos.
RLS - remove C++ style comments.
RLS - update row_security GUC description.
RLS - update CreateRole and AlterRole error message for BYPASSRLS.
RLS - update row-security policy functions and structures.

* Refactor RowSecurityEntry.
* Add permission checks.
* Modify lookup key to use only policy name and table.
* Modify AlterPolicy to use optional roles and command.
  - If a role is not provided, then the current value is left as is.
  - If a command is not provided, then the current value is left as is.
RLS - update regression tests.