Run(): simplify runCopyStdio()
Run(): simplify runCopyStdio()

Rework runCopyStdio() to remove the "reading" variable, and to reduce
cyclomatic complexity.

Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>

Closes: #795
Approved by: rhatdan

TomSweeneyRedHat/buildah,nalind/buildah,rhatdan/buildah,projectatomic/buildah,rhatdan/buildah,nalind/buildah-1,ipbabble/buildah,rhatdan/buildah,TomSweeneyRedHat/buildah,projectatomic/buildah,ipbabble/buildah,nalind/buildah-1,nalind/buildah,TomSweeneyRedHat/buildah,nalind/buildah-1,projectatomic/buildah