security label support, part.1

The attached patch is a part of efforts to support security label
on database objects.

It adds a new system catalog named pg_seclabel, that has similar
structure with pg_description.

  #define SecLabelRelationId        3037

  CATALOG(pg_seclabel,3037) BKI_WITHOUT_OIDS
  {
      Oid         reloid;     /* OID of table containing the object */
      Oid         objoid;     /* OID of the object itself */
      int4        subid;      /* column number, or 0 if not used */
      text        label;      /* security label of the object */
  } FormData_pg_seclabel;

This patch provides only its definition and basic internal APIs to
get/set/delete security labels, so, we also need to apply the part.2
patch to support ALTER statement to manage security labels, in addition
to the part.1.

Right now, modular sepgsql will perform on the patch.
It can be checked out using:
  % svn checkout http://sepgsql.googlecode.com/svn/trunk/sepgsql

Thanks,
--
KaiGai Kohei <kaigai@ak.jp.nec.com>