portmapper: change userlandProxyCommandName to a const

it's not overridden anywhere, so may as well be a const

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

libnetwork/portmapper/proxy_linux.go

@@ -11,7 +11,7 @@ import (
 	"time"
 )
 
-var userlandProxyCommandName = "docker-proxy"
+const userlandProxyCommandName = "docker-proxy"
 
 func newProxyCommand(proto string, hostIP net.IP, hostPort int, containerIP net.IP, containerPort int, proxyPath string) (userlandProxy, error) {
 	path := proxyPath