Print devicemapper status details in docker info

This adds a generic Status call in the Driver api and
implements if for the devicemapper backend.

The status is an array of key/value strings rather than a map so that
we can guarantee some static order of the docker info output.

server.go

@@ -376,6 +376,7 @@ func (srv *Server) DockerInfo() *APIInfo {
 		Containers:         len(srv.runtime.List()),
 		Images:             imgcount,
 		Driver:             srv.runtime.driver.String(),
+		DriverStatus:       srv.runtime.driver.Status(),
 		MemoryLimit:        srv.runtime.capabilities.MemoryLimit,
 		SwapLimit:          srv.runtime.capabilities.SwapLimit,
 		IPv4Forwarding:     !srv.runtime.capabilities.IPv4ForwardingDisabled,