rc: enable go profiling by default on the --rc port

This means you can use the pprof tool on a running rclone, eg

    go tool pprof http://localhost:5572/debug/pprof/heap

fs/rc/rc.go

@@ -11,6 +11,7 @@ import (
 	"encoding/json"
 	"io"
 	"net/http"
+	_ "net/http/pprof" // install the pprof http handlers
 	"strings"
 
 	"github.com/ncw/rclone/cmd/serve/httplib"