http: s/TINY_INITIAL_POST_SIZE/EXPECT_100_THRESHOLD

Make the name reflect its use better, and add a short comment describing
what it's for.
diff --git a/lib/http.c b/lib/http.c
index e611214..319a819 100644
--- a/lib/http.c
+++ b/lib/http.c
@@ -2600,7 +2600,7 @@
       data->state.expect100header =
         Curl_compareheader(ptr, "Expect:", "100-continue");
     }
-    else if(postsize > TINY_INITIAL_POST_SIZE || postsize < 0) {
+    else if(postsize > EXPECT_100_THRESHOLD || postsize < 0) {
       result = expect100(data, conn, req_buffer);
       if(result)
         return result;
diff --git a/lib/http.h b/lib/http.h
index 7ce4bd9..a845f56 100644
--- a/lib/http.h
+++ b/lib/http.h
@@ -7,7 +7,7 @@
  *                            | (__| |_| |  _ <| |___
  *                             \___|\___/|_| \_\_____|
  *
- * Copyright (C) 1998 - 2015, Daniel Stenberg, <daniel@haxx.se>, et al.
+ * Copyright (C) 1998 - 2017, Daniel Stenberg, <daniel@haxx.se>, et al.
  *
  * This software is licensed as described in the file COPYING, which
  * you should have received as part of this distribution. The terms
@@ -115,8 +115,13 @@
 #define MAX_INITIAL_POST_SIZE (64*1024)
 #endif
 
-#ifndef TINY_INITIAL_POST_SIZE
-#define TINY_INITIAL_POST_SIZE 1024
+/* EXPECT_100_THRESHOLD is the request body size limit for when libcurl will
+ * automatically add an "Expect: 100-continue" header in HTTP requests. When
+ * the size is unknown, it will always add it.
+ *
+ */
+#ifndef EXPECT_100_THRESHOLD
+#define EXPECT_100_THRESHOLD 1024
 #endif
 
 #endif /* CURL_DISABLE_HTTP */