ocserv: init at 0.12.1 (#42871)

`ocserv` is a VPN server which follows the openconnect protocol
(https://github.com/openconnect/protocol). The packaging is slightly
inspired by the AUR version
(https://aur.archlinux.org/packages/ocserv/).

This patch initializes the package written in C, the man pages and a
module for a simple systemd unit to run the VPN server. The package
supports the following authentication methods for the server:

* `plain` (mostly username/password)
* `pam`

The third method (`radius`) is currently not supported since `nixpkgs`
misses a packaged client.

The module can be used like this:

``` nix
{
  services.ocserv = {
    enable = true;
    config = ''
      ...
    '';
  };
}
```

The option `services.ocserv.config` is required on purpose to
ensure that nobody just enables the service and experiences unexpected
side-effects on the system. For a full reference, please refer to the
man pages, the online docs or the example value.

The docs recommend to simply use `nobody` as user, so no extra user has
been added to the internal user list. Instead a configuration like
this can be used:

```
run-as-user = nobody
run-as-group = nogroup
```

/cc @tenten8401
Fixes #42594