services/postgresql: update to the latest upstream nixos module

[lf-]

I want a newish postgres config to work on my Mac.

This contains a little hack (putting the check-config derivation into the start script as a comment) to deal with not having #356. Also, it does not implement the stuff that requires a postStart script.

It's otherwise identical to the upstream module.

I changed the service to use utf8 always because the charset option got deleted in the upstream module. It seems to work and I can connect to it.