Make 'docker build' send non-err output to stdout

Right now 'docker build' will send:
   Sending build context to Docker daemon
to stderr, instead of stdout.  This PR fixes that.

I looked in the rest of api/client/commands.go for other cases
that might do this and only one jumped out at me:
  https://github.com/docker/docker/blob/master/api/client/commands.go#L2202
but I think if I changed that to go to stdout then it'll mess people up
who are expecting just the container ID to be printed to the screen and
there is no --quiet type of flag we can check.

Closes #9404

Signed-off-by: Doug Davis <dug@us.ibm.com>
Is there another function I should use instead? I looked for one that returned the stderr separately from the stdout but didn't notice one - but perhaps I missed it.