Ignore ErrClosedPipe for stdin in Container.Attach.

But pass `err` in the error channel anyway; don't ignore it anymore.
Merge pull request #2383 from jpetazzo/cleanup-errclosedpipe-for-stdin

Ignore ErrClosedPipe for stdin in Container.Attach.
Merge branch 'master' into 2328-docker-events-since-date-string