Make SessionHandle keep record if it is used with the multi interface or

the easy interface, it CANNOT be used by a mixture.

lib/multi.c

@@ -263,10 +263,13 @@ CURLMcode curl_multi_perform(CURLM *multi_handle, int *running_handles)
     case CURLM_STATE_INIT:
       /* init this transfer. */
       easy->result=Curl_pretransfer(easy->easy_handle);
+
       if(CURLE_OK == easy->result) {
         /* after init, go CONNECT */
         easy->state = CURLM_STATE_CONNECT;
         result = CURLM_CALL_MULTI_PERFORM; 
+        
+        easy->easy_handle->state.used_interface = Curl_if_multi;
       }
       break;
     case CURLM_STATE_CONNECT:

lib/transfer.c

@@ -1156,6 +1156,8 @@ CURLcode Curl_perform(struct SessionHandle *data)
   struct connectdata *conn=NULL;
   char *newurl = NULL; /* possibly a new URL to follow to! */
 
+  data->state.used_interface = Curl_if_easy;
+
   res = Curl_pretransfer(data);
   if(res)
     return res;

lib/urldata.h

@@ -488,6 +488,12 @@ typedef enum {
 #define MAX_CURL_PASSWORD_LENGTH 256
 
 struct UrlState {
+  enum {
+    Curl_if_none,
+    Curl_if_easy,
+    Curl_if_multi
+  } used_interface;
+
   /* buffers to store authentication data in, as parsed from input options */
   char user[MAX_CURL_USER_LENGTH];
   char passwd[MAX_CURL_PASSWORD_LENGTH];