fleetd: fix support for "fleetd version"
fleetd: fix support for "fleetd version"

Commit 32a51814 broke fleetd, making "fleetd version" the *only* thing it
could do and causing a index error panic otherwise. Now in addition to
properly checking the number of unparsed args exit with a usage error if
given any other unknown arguments.

Simply exiting if there are any unparsed args would have been sufficient
to fix #1210 but on the other hand allowing "fleetd version" doesn't
hurt so might as well keep it.

Fixes #1214 and #1210

dalbani/fleet,jonboulle/fleet,mwitkow-io/fleet,bviswana101/fleet,dongsupark/fleet,mookjp/fleet,WPMedia/fleet,giantswarm/fleet,coreos/fleet,jonboulle/fleet,mookjp/fleet,coreos/fleet,wuqixuan/fleet,thepwagner/fleet,dongsupark/fleet,thepwagner/fleet,improbable-io/fleet,bviswana101/fleet,endocode/fleet,mischief/fleet,mwitkow-io/fleet,giantswarm/fleet,endocode/fleet,WPMedia/fleet,endocode/fleet,octoblu/fleet,paveltiunov/fleet,flandr/fleet,webwlsong/fleet,mwitkow/fleet,esatterwhite/fleet,dalbani/fleet,esatterwhite/fleet,webwlsong/fleet,flandr/fleet,jonboulle/fleet,mwhooker/fleet,octoblu/fleet,mischief/fleet,giantswarm/fleet,mwitkow/fleet,paveltiunov/fleet,improbable-io/fleet,dongsupark/fleet,dalbani/fleet,coreos/fleet,improbable-io/fleet,mwhooker/fleet,wuqixuan/fleet