transfer: return without select when the read loop reached maxcount

Regression added in 790d6de485. The was then added to avoid one
particular transfer to starve out others. But when aborting due to
reading the maxcount, the connection must be marked to be read from
again without first doing a select as for some protocols (like SFTP/SCP)
the data may already have been read off the socket.

Reported-by: Dan Donahue
Bug: https://curl.haxx.se/mail/lib-2016-07/0057.html

lib/transfer.h

@@ -40,10 +40,9 @@ typedef enum {
 
 CURLcode Curl_follow(struct Curl_easy *data, char *newurl,
                      followtype type);
-
-
 CURLcode Curl_readwrite(struct connectdata *conn,
-                        struct Curl_easy *data, bool *done);
+                        struct Curl_easy *data, bool *done,
+                        bool *comeback);
 int Curl_single_getsock(const struct connectdata *conn,
                         curl_socket_t *socks,
                         int numsocks);