[proxy] Renaming flags: -H -> -D, -L -> -H

As per discussion in issue 875

weave

@@ -28,7 +28,7 @@ usage() {
     echo "weave launch       [-password <password>] [-nickname <nickname>] [-nodiscovery]"
     echo "                       [-iprange <cidr>] <peer> ..."
     echo "weave launch-dns   <cidr>"
-    echo "weave launch-proxy [-H <docker_endpoint>] [-L <address>] [--with-dns] [--with-ipam]"
+    echo "weave launch-proxy [-D <docker_endpoint>] [-H <address>] [--with-dns] [--with-ipam]"
     echo "weave connect      [--replace] [<peer> ...]"
     echo "weave forget       <peer> ..."
     echo "weave run          [--with-dns] [<cidr> ...] <docker run args> ..."