nixos/postgresql: support 0750 for data directory (#65245)

* 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.

* postgresql: allow changing initidb arguments via module system

Closes https://github.com/NixOS/nixpkgs/issues/18829

+ some cleanups

* addressed review comments and some fixes

* whoops

* change groupAccess to tristate, to not force `chmod` on dataDir.

Making mask either 0700 or 0750 is too restrictive..

* WIP

* let's not support group mode for versions pre-11.

The only fix is to change mode to 0700 before start, because otherwise postgresql
doesn't start, and error is non-obvious.

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