transfer: make Expect: 100-continue timeout configurable.

Replaced the #define CURL_TIMEOUT_EXPECT_100 in transfer.c with the
CURLOPT_EXPECT_100_TIMEOUT_MS option to make the timeout configurable.

lib/urldata.h

@@ -1596,6 +1596,8 @@ struct UserDefined {
 
   bool ssl_enable_npn;  /* TLS NPN extension? */
   bool ssl_enable_alpn; /* TLS ALPN extension? */
+
+  long expect_100_timeout; /* in milliseconds */
 };
 
 struct Names {