In pg_dump, dump SEQUENCE SET items in the data not pre-data section.

Represent a sequence's current value as a separate TableDataInfo dumpable
object, so that it can be dumped within the data section of the archive
rather than in pre-data.  This fixes an undesirable inconsistency between
the meanings of "--data-only" and "--section=data", and also fixes dumping
of sequences that are marked as extension configuration tables, as per a
report from Marko Kreen back in July.  The main cost is that we do one more
SQL query per sequence, but that's probably not very meaningful in most
databases.

Back-patch to 9.1, since it has the extension configuration issue even
though not the --section switch.

doc/src/sgml/ref/pg_dump.sgml

@@ -739,11 +739,11 @@ PostgreSQL documentation
           sections. The default is to dump all sections.
          </para>
          <para>
-          The data section contains actual table data as well as large-object
-          definitions.
-          Post-data items consist of definitions of indexes, triggers, rules
+          The data section contains actual table data, large-object
+          contents, and sequence values.
+          Post-data items include definitions of indexes, triggers, rules,
           and constraints other than validated check constraints.
-          Pre-data items consist of all other data definition items.
+          Pre-data items include all other data definition items.
          </para>
        </listitem>
      </varlistentry>