Teach pg_size_pretty and pg_size_bytes about petabytes

Teach pg_size_pretty and pg_size_bytes about petabytes

There was talk about adding units all the way up to yottabytes but it
seems quite far-fetched that anyone would need those.  Since such large
units are not exactly commonplace, it seems unlikely that having
pg_size_pretty outputting unit any larger than petabytes would actually be
helpful to anyone.

Since petabytes are on the horizon, let's just add those only.  Maybe one
day we'll get to add additional units, but it will likely be a while
before we'll need to think beyond petabytes in regards to the size of a
database.

Author: David Christensen
Discussion: https://postgr.es/m/CAOxo6XKmHc_WZip-x5QwaOqFEiCq_SVD0B7sbTZQk+qqcn2qaw@mail.gmail.com