fix usage for completly deprecated flag

Docker-DCO-1.1-Signed-off-by: Victor Vieux <victor.vieux@docker.com> (github: vieux)
fix panic with only long flags or only one deprecatd

Docker-DCO-1.1-Signed-off-by: Victor Vieux <victor.vieux@docker.com> (github: vieux)
Merge pull request #4476 from vieux/improve_flags

Improve flags