add recursive device nodes

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
update test

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
Merge pull request #5995 from vieux/recur_nodes

Add device nodes recursively
Fix add hang when dest is .

Fixes #3960
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)