Manager shut down gracefully w/ SIGTERM
Manager shut down gracefully w/ SIGTERM

Adds an import of the syscall library. Might break Windows support? Does
swarmd even support Windows?

closes #725

Signed-off-by: Drew Erny <83d780db39fae693cf18623ace93c7ec0dc91590@docker.com>

sungwonh/swarmkit,cezarsa/swarmkit,dperny/swarmkit-1,RenaudWasTaken/swarmkit,cheyang/swarmkit,runshenzhu/swarmkit,elifa/swarmkit,AliyunContainerService/swarmkit,dperny/swarmkit-1,runshenzhu/swarmkit,Noyan759/swarmkit,docker/swarmkit,cheyang/swarmkit,allencloud/swarmkit,AliyunContainerService/swarmkit,docker/swarmkit,yp-engineering/swarmkit,cezarsa/swarmkit,deljuven/swarmkit,aaronlehmann/swarmkit,fate-grand-order/swarmkit,elifa/swarmkit,diogomonica/swarmkit,alyyousuf7/swarmkit,yp-engineering/swarmkit,deljuven/swarmkit,aaronlehmann/swarmkit,Noyan759/swarmkit,alyyousuf7/swarmkit,allencloud/swarmkit,yongtang/swarmkit,diogomonica/swarmkit,vasil-yordanov/swarmkit,jlhawn/swarmkit,vasil-yordanov/swarmkit,sungwonh/swarmkit,fate-grand-order/swarmkit,yongtang/swarmkit,jlhawn/swarmkit,RenaudWasTaken/swarmkit