Backport the new BackgroundPsql modules and the constructor functions,
background_psql() and interactive_psql, to all supported
branches. That makes it easier to backpatch tests that use it.
BackgroundPsql was introduced in version 16. On version 16, this
commit backports just the new timeout argument from master (commit
334f512f45). On older branches, the whole facility. This includes the
change to `use warnings FATAL => 'all'`, which we haven't otherwise
backported, but it seems good to keep the file identical across
branches.
Discussion: https://www.postgresql.org/message-id/b7c64f20-ea01-4f15-9088-0cd6832af149@iki.fi
187b8991f7