--help option and help command should print to stdout not stderr

--help and help are successfull commands so output should not go to
error.

QE teams have requested this change, also users doing docker help | less
or docker run --help | less would expect this to work.

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)
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