Unlink old socket if one is present
Unlink old socket if one is present

The manager would exit with

    Error: listen unix /tmp/swarm.sock: bind: address already in use

after an unclean shutdown.

It seems better to remove the old socket than failing like this.

Signed-off-by: Aaron Lehmann <8ecfc6017a87905413dcd7d63696a2a4c351b604@docker.com>

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