pg_upgrade: check for inconsistencies in not-null constraints w/inheritance

With tables defined like this,
  CREATE TABLE ip (id int PRIMARY KEY);
  CREATE TABLE ic (id int) INHERITS (ip);
  ALTER TABLE ic ALTER id DROP NOT NULL;

pg_upgrade fails during the schema restore phase due to this error:
  ERROR: column "id" in child table must be marked NOT NULL

This can only be fixed by marking the child column as NOT NULL before
the upgrade, which could take an arbitrary amount of time (because ic's
data must be scanned).  Have pg_upgrade's check mode warn if that
condition is found, so that users know what to adjust before running the
upgrade for real.

Author: Ali Akbar <the.apaan@gmail.com>
Reviewed-by: Justin Pryzby <pryzby@telsasoft.com>
Backpatch-through: 13
Discussion: https://postgr.es/m/CACQjQLoMsE+1pyLe98pi0KvPG2jQQ94LWJ+PTiLgVRK4B=i_jg@mail.gmail.com

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