nixos/nsswitch: Make databases more configurable

Instead of hardcoding all nss modules that are added into nsswitch,
there are now options exposed.
This allows users to add own nss modules (I had this issue with
winbindd, for example).
Also, nss modules could be moved to their NixOS modules which would
make the nsswitch module slimmer.

As the lists are now handled by the modules system, we can use mkOrder
to ensure a proper order as well as mkForce to override one specific
database type instead of the entire file.