nixos/sshd: validate ssh configs during build

With `sshd -t` config validation for SSH is possible. Until now, the
config generated by Nix was applied without any validation (which is
especially a problem for advanced config like `Match` blocks).

When deploying broken ssh config with nixops to a remote machine it gets
even harder to fix the problem due to the broken ssh that makes reverts
with nixops impossible.

This change performs the validation in a Nix build environment by
creating a store path with the config and generating a mocked host key
which seems to be needed for the validation. With a broken config, the
deployment already fails during the build of the derivation.

The original attempt was done in NixOS#56345 by adding a submodule for Match
groups to make it harder screwing that up, however that made the module
far more complex and config should be described in an easier way as
described in NixOS/rfcs#42.

nixos/modules/services/networking/ssh/sshd.nix

@@ -4,6 +4,15 @@ with lib;
 
 let
 
+  sshconf = pkgs.runCommand "sshd.conf-validated" { nativeBuildInputs = [ cfgc.package ]; } ''
+    cat >$out <<EOL
+    ${cfg.extraConfig}
+    EOL
+
+    ssh-keygen -f mock-hostkey -N ""
+    sshd -t -f $out -h mock-hostkey
+  '';
+
   cfg  = config.services.openssh;
   cfgc = config.programs.ssh;
 
@@ -339,7 +348,7 @@ in
 
     environment.etc = authKeysFiles //
       { "ssh/moduli".source = cfg.moduliFile;
-        "ssh/sshd_config".text = cfg.extraConfig;
+        "ssh/sshd_config".source = sshconf;
       };
 
     systemd =