fleetd: improve usage message and cmdline parsing
fleetd: improve usage message and cmdline parsing

It's a common mistake for users to accidentally invoke fleet
(which is, unfortunately, a symlink to fleetd on CoreOS) instead of
fleetctl, which at best is confusing and at worst can cause problems
if fleetd is accidentally started up somewhere it shouldn't be and
starts doing work undesirably.

This was already fixed by another commit which made it backwards
incompatible. If anyone happened to be invoking fleetd with arguments,
then arguably in that case they were doing it wrong anyway (since
fleetd would just ignore them) and so this provides better behaviour.

The current patch just improves the usage message and lets the users
know if they were trying for fleetctl.

Commit-log-by: Jonathan Boulle <0d47477beef1d882e8812885f3dcf66c4a47c346@gmail.com>

endocode/fleet,improbable-io/fleet,jonboulle/fleet,improbable-io/fleet,giantswarm/fleet,endocode/fleet,dalbani/fleet,dongsupark/fleet,coreos/fleet,coreos/fleet,dalbani/fleet,dongsupark/fleet,improbable-io/fleet,jonboulle/fleet,giantswarm/fleet,coreos/fleet,giantswarm/fleet,jonboulle/fleet,dongsupark/fleet,endocode/fleet,dalbani/fleet