List offending databases in pg_upgrade datallowconn check

List offending databases in pg_upgrade datallowconn check

The check for datallowconn being properly set on all databases in the
old cluster errored out on the first instance, rather than report the
set of problematic databases.  This adds reporting to a textfile like
how many other checks are performed.  While there, also add a comment
to the function as per how other checks are commented.

This check won't catch if template1 isn't allowing connections, since
that's used for connecting in the first place.  That error remains as
it is today:

connection to server on socket ".." failed: FATAL:  database "template1" is not currently accepting connections

Author: Jeevan Ladhe <jeevan.ladhe@enterprisedb.com>
Reviewed-by: Suraj Kharage <suraj.kharage@enterprisedb.com>
Reviewed-by: Justin Pryzby <pryzby@telsasoft.com>
Discussion: https://postgr.es/m/CAOgcT0McABqF_iFFQuuRf9is+gmYMsmUu_SWNikyr=2VGyP9Jw@mail.gmail.com