curl: timeout in the read callback

The read callback can timeout if there's nothing to read within the
given maximum period. Example use case is when doing "curl -m 3
telnet://example.com" or anything else that expects input on stdin or
similar that otherwise would "hang" until something happens and then not
respect the timeout.

This fixes KNOWN_BUG 8.1, first filed in July 2009.

Bug: https://sourceforge.net/p/curl/bugs/846/

Closes #9815

src/tool_operate.h

@@ -37,6 +37,7 @@ struct per_transfer {
   long retry_numretries;
   long retry_sleep_default;
   long retry_sleep;
+  struct timeval start; /* start of this transfer */
   struct timeval retrystart;
   char *this_url;
   unsigned int urlnum; /* the index of the given URL */