calling `docker --badopt` doesn't show cli plugins

[SvenDowideit]

somehow docker --badopt shows a different list of available commands from docker --help - and does not include the cli plugins installed.

(I found this because i wanted to see if i could make a docker---all plugin, but for now, that seems fobidden.)

@ijc

[ijc]

I confirm that I see this too. I'll look into a fix.