psql \dP: list partitioned tables and indexes

psql \dP: list partitioned tables and indexes

The new command lists partitioned relations (tables and/or indexes),
possibly with their sizes, possibly including partitioned partitions;
their parents (if not top-level); if indexes show the tables they belong
to; and their descriptions.

While there are various possible improvements to this, having it in this
form is already a great improvement over not having any way to obtain
this report.

Author: Pavel Stěhule, with help from Mathias Brossard, Amit Langote and
	Justin Pryzby.
Reviewed-by: Amit Langote, Mathias Brossard, Melanie Plageman,
	Michaël Paquier, Álvaro Herrera