checksrc: detect fopen() for text without the FOPEN_* macros

Follow-up to e8423f9ce1 with discussionis in
https://github.com/bagder/curl/pull/258

This check scans for fopen() with a mode string without 'b' present, as
it may indicate that an FOPEN_* define should rather be used.

src/checksrc.whitelist

@@ -1,3 +1,3 @@
  * 'name=@filename,filename2,filename3'
  * 'name=@filename;type=image/gif,filename2,filename3'
- 
+  file = fopen(name, "r"); /* VMS */

src/tool_operate.c

@@ -151,7 +151,7 @@ static curl_off_t vms_realfilesize(const char * name,
   int ret_stat;
   FILE * file;
 
-  file = fopen(name, "r");
+  file = fopen(name, "r"); /* VMS */
   if(file == NULL) {
     return 0;
   }