This is analogous to the syntax allowed for VACUUM. This allows us to
avoid making new options reserved keywords and makes it easier to
allow arbitrary argument order. Oh, and it's consistent with the other
commands, too.

Author: Nathan Bossart
Reviewed-By: Michael Paquier, Masahiko Sawada
Discussion: https://postgr.es/m/D3FC73E2-9B1A-4DB4-8180-55F57D116B4E@amazon.com