nixos/postgresql: support 0750 for data directory

This is rework of part of https://github.com/NixOS/nixpkgs/pull/46670.
My usecase was to be able to inspect PG datadir as wheel user.

PG11 now allows starting server with 0750 mask for data dir.
`groupAccess = true` now does this automatically. The only thing you have to do
is to set group ownership.

For PG10 and below, I've described a hack how this can be done. Before this PR
hack was impossible. The hack isn't ideal, because there is short
period of time when dir mode is 0700, so I didn't want to make it official.

Test/example is present too.