pg_dump: Add --sequence-data.

pg_dump: Add --sequence-data.

This new option instructs pg_dump to dump sequence data when the
--no-data, --schema-only, or --statistics-only option is specified.
This was originally considered for commit a7e5457db8, but it was
left out at that time because there was no known use-case.  A
follow-up commit will use this to optimize pg_upgrade's file
transfer step.

Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Discussion: https://postgr.es/m/Zyvop-LxLXBLrZil%40nathan