flexnbd: Split the proxy mode out into its own binary.

"flexnbd-proxy ..." should be identical in operation to "flexnbd proxy ..."

tests/acceptance/flexnbd.rb

@@ -242,7 +242,7 @@ module FlexNBD
     end
 
     def proxy_cmd( connect_ip, connect_port )
-      "#{bin} proxy "\
+      "#{bin}-proxy "\
         "--addr #{ip} "\
         "--port #{port} "\
         "--conn-addr #{connect_ip} "\