Add Wait() calls in the appropriate spots

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
libcontainer/nsinit: remove Wait call from Exec and Kill from Attach in tty_term.go

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
Merge pull request #6030 from erikh/5971-fix-zombies

Add Wait() calls in the appropriate spots