[v9.2] SECURITY LABEL on shared database object

[I noticed my new address was not registered to pgsql-hackers, so try again]

The attached patch enables to assign security labels of shared database object
types (databases, tablespaces, roles).
Like as pg_shdescription doing, it adds a new pg_shseclabel catalog corresponding
to the existing pg_seclabel catalog. Its internal APIs are kept; if and when
(Get|Set|Delete)SecurityLabel is invoked for shared catalogs, it references the
pg_shseclabel instead of the pg_seclabel.

This patch also contains pg_dump support, use cases of contrib/sepgsql, regression
test and updates of sgml documentation.

Thanks,
--
NEC Europe Ltd, SAP Global Competence Center
KaiGai Kohei <kohei.kaigai@eu.nec.com>