pgsql: Find invalid databases during upgrade check stage

Find invalid databases during upgrade check stage

Before continuing with the check start by checking that all databases
allow connections to avoid a hard fail without proper error reporting.

Inspired by a larger patch by Thomas Krennwallner.

Discussion: /messages/by-id/f9315bf0-e03e-4490-9f0d-5b6f7a6d9908@postsubmeta.net

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f638aafd1ea8505750ca9546e0e0b2e4cf31027d

Modified Files
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src/bin/pg_upgrade/check.c | 37 +++++++++++++++++++++-------------
src/bin/pg_upgrade/t/002_pg_upgrade.pl | 2 +-
2 files changed, 24 insertions(+), 15 deletions(-)