vcgencmd: tweak usage

[ghost]

This PR depends on #663 also integrating, since it adds mention of the existence of a man page to the usage message (basic help output).

Tweaks are:

1. Don't mention vcgencmd commands - not needed now we have a man page.
2. Mention the man page in usage message, since it's a new addition, for improved discoverability.
3. Tweak what happens in the case where the user tried to invoke a gencmd that doesn't exist: also display usage.