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Use "dpkg-divert" on our sample configuration file (instead of just replacing it)

This should stop our changes from getting overwritten when folks do weird things like upgrade PostgreSQL inside the image (which isn't a good idea, but this change is still more correct anyhow).

Update "gpg" invocations to use "--batch"

See https://bugs.debian.org/913614

Closes #512

Utilize "initdb" functionality better to allow "POSTGRES_INITDB_ARGS=--auth-local=md5"

This also closes a slight bug we've had previously where the "postgres" user is _always_ created (now we only create the user specified via the environment variables).

Update psql invocations to properly escape user input!

See also:

- https://stackoverflow.com/a/18683163/433558
- https://www.postgresql.org/docs/9.3/static/app-psql.html#APP-PSQL-VARIABLES
- https://www.postgresql.org/docs/9.3/static/app-psql.html#APP-PSQL-INTERPOLATION

Add "stretch-backports" to 11+ for clang-6.0

(also, remove *.pyc files for size)

Fix Debian "postgres" user HOME (to match Debian package)

See https://salsa.debian.org/postgresql/postgresql-common/blob/997d842ee744687d99a2b2d95c1083a2615c79e8/debian/postgresql-common.postinst#L32-35 for the bit of the upstream Debian package which sets this.