add \dpS to psql

Hi hackers,

As discussed elsewhere [0], \dp doesn't show privileges on system objects,
and this behavior is not mentioned in the docs.  I've attached a small
patch that adds support for the S modifier (i.e., \dpS) and the adjusts the
docs.

Thoughts?

[0] https://postgr.es/m/a2382acd-e465-85b2-9d8e-f9ed1a5a66e9%40postgrespro.ru

-- 
Nathan Bossart
Amazon Web Services: https://aws.amazon.com