Print usage when user attempts to use `mount` with free args
Print usage when user attempts to use `mount` with free args

Missing '_' in signature wasn't enough to prevent the user from trying to use the command with a free argument, e.g. `mount gdrive`. The command would behave as if no free args were provided at all, that is print a list of mounted systems.

chromium/axiom,rpaquay/axiom,mcanthony/axiom