networking: Restore the original /etc/hosts on activation

A patch that replaced the original file with a symlink to nix store was
reverted because MacOS Network framework doesn't support symlinks for
the file.

The revert leaves the system without any /etc/hosts file at all though.
To fix this, an activation step is added to restore the original file
from .before-nix-darwin backup, if it exists.

Signed-off-by: Ihar Hrachyshka <ihar.hrachyshka@gmail.com>

modules/networking/default.nix

@@ -132,6 +132,11 @@ in
       ${optionalString (cfg.wakeOnLan.enable != null) ''
         systemsetup -setWakeOnNetworkAccess '${onOff cfg.wakeOnLan.enable}' &> /dev/null
       ''}
+
+      if [ -e /etc/hosts.before-nix-darwin ]; then
+        echo "restoring /etc/hosts..." >&2
+        sudo mv /etc/hosts{.before-nix-darwin,}
+      fi
     '';
 
   };