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

--help and help are successful 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.

Usage statement should only be printed when the user asks for it.
Errors should print error message and then suggest the docker COMMAND --help
command to see usage information.

The current behaviour causes the user to have to search for the error message
and sometimes scrolls right off the screen.  For example a error on a
"docker run" command is very difficult to diagnose.

Finally erros should always exit with a non 0 exit code, if the user
makes a CLI error.

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)
Merge pull request #6052 from rhatdan/help

--help option and help command should print to stdout not stderr
Update /etc/hosts when linked container is restarted

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
pkg/graphdb: tests for Parents and Children

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
pkg/networkfs/etchosts: tests for Update method

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
Update networking doc with clarified text regarding links usage

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)