nbd_install help: usbinput is obsolete

This has been deprecated and replaced by the keyboard hook long ago.
nbd-client: use new -systemd-mark / -m switch

Installing the file prefixed with @ was a workaround to make sure that
systemd doesn't kill the process on shutdown. nbd-client now has a
switch for this built in.

see also a5f9bb202817a76e62bd86d846666dd3ca1868fd