[PATCH] expand the units that pg_size_pretty supports on output

Hi folks,

Enclosed is a patch that expands the unit output for
pg_size_pretty(numeric) going up to Yottabytes; I reworked the existing
numeric output code to account for the larger number of units we're using
rather than just adding nesting levels.

There are also a few other places that could benefit from expanded units,
but this is a standalone starting point.

Best,

David