networkmanager: Add option for appending DNS settings

If the option is enabled, the DNS servers from networking.nameservers
will be inserted in /etc/resolv.conf after the DNS servers that
NetworkManager receieves by DHCP, or that is configured manually
in the connection settings.
Manual: Describe NixOS package management styles
Manual: Document user management
Manual: Document X11 configuration
Manual: Document LUKS encryption
Manual: Remove "bleeding edge" remark
Manual: Add section on how to do rollbacks
nixos-help: Use xdg-open if available
Manual: Move NVIDIA stuff into its own subsection
Manual: Document synaptics support
Manual: "filesystem" -> "file system"
Manual: Kernel configuration
Manual: Nix store maintenance
Manual: Document how to repair a corrupted Nix store
Manual: Document how to deal with binary cache issues
try to fix zfs into initrd