Add --section option to pg_dump and pg_restore.

Add --section option to pg_dump and pg_restore.

Valid values are --pre-data, data and post-data. The option can be
given more than once. --schema-only is equivalent to
--section=pre-data --section=post-data. --data-only is equivalent
to --section=data.

Andrew Dunstan, reviewed by Joachim Wieland and Josh Berkus.