Add 'grdctl' command line tool

With Settings going RDP-only, it'll be useful to have a way to still configure VNC (and RDP); so add grdctl. It looks like this:

$ grdctl --help
Usage: grdctl [OPTIONS...] COMMAND [SUBCOMMAND]...
Commands:
  rdp - RDP subcommands:
    enable                                   - Enable the RDP backend
    disable                                  - Disable the RDP backend
    set-tls-cert <path-to-cert>              - Set path to TLS certificate
    set-tls-key <path-to-key>                - Set path to TLS key
    set-credentials <username> <password>    - Set username and password credentials
    clear-credentials                        - Clear username and password credentials
    enable-view-only                         - Disable remote control of input devices
    disable-view-only                        - Enable remote control of input devices

  vnc - VNC subcommands:
    enable                                   - Enable the VNC backend
    disable                                  - Disable the VNC backend
    set-password <password>                  - Set the VNC password
    clear-password                           - Clear the VNC password
    set-auth-method password|prompt          - Set the authorization method
    enable-view-only                         - Disable remote control of input devices
    disable-view-only                        - Enable remote control of input devices

  status - Show current status

Options:
  --help                                     - Print this help text

and

$ grdctl status
RDP:
	Status: enabled
	TLS certificate: /path/to/tls.crt
	TLS key: /path/to/tls.key
	View-only: no
	Username: (hidden)
	Password: (hidden)
VNC:
	Status: enabled
	Auth method: prompt
	View-only: yes
	Password: (empty)

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Marked as draft as it includes !71 (merged).