serve: make the servers self registering

This is so that they can import cmd/serve without causing an import
loop.

The active servers can now be configured by commenting lines out in
cmd/all/all.go like all the other commands.

cmd/serve/ftp/ftp.go

@@ -18,6 +18,7 @@ import (
 	"time"
 
 	"github.com/rclone/rclone/cmd"
+	"github.com/rclone/rclone/cmd/serve"
 	"github.com/rclone/rclone/cmd/serve/proxy"
 	"github.com/rclone/rclone/cmd/serve/proxy/proxyflags"
 	"github.com/rclone/rclone/fs"
@@ -88,6 +89,7 @@ func init() {
 	vfsflags.AddFlags(Command.Flags())
 	proxyflags.AddFlags(Command.Flags())
 	AddFlags(Command.Flags())
+	serve.Command.AddCommand(Command)
 }
 
 // Command definition for cobra