DISABLE_PAGE_SKIPPING is available since v9.6, and SKIP_LOCKED since
v12.  They lacked equivalents for vacuumdb, so this closes the gap.

Author: Nathan Bossart
Reviewed-by: Michael Paquier, Masahiko Sawada
Discussion: https://postgr.es/m/FFE5373C-E26A-495B-B5C8-911EC4A41C5E@amazon.com