headers: forward declare CURL, CURLM and CURLSH as structs

Instead of typedef'ing to void, typedef to their corresponding actual
struct names to allow compilers to type-check.

Assisted-by: Reinhard Max

include/curl/curl.h

@@ -91,7 +91,7 @@
 extern "C" {
 #endif
 
-typedef void CURL;
+typedef struct SessionHandle CURL;
 
 /*
  * libcurl external API function linkage decorations.
@@ -2258,7 +2258,7 @@ typedef void (*curl_unlock_function)(CURL *handle,
                                      curl_lock_data data,
                                      void *userptr);
 
-typedef void CURLSH;
+typedef struct Curl_share CURLSH;
 
 typedef enum {
   CURLSHE_OK,  /* all is fine */

include/curl/multi.h

@@ -7,7 +7,7 @@
  *                            | (__| |_| |  _ <| |___
  *                             \___|\___/|_| \_\_____|
  *
- * Copyright (C) 1998 - 2015, Daniel Stenberg, <daniel@haxx.se>, et al.
+ * Copyright (C) 1998 - 2016, 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
@@ -52,7 +52,7 @@
 extern "C" {
 #endif
 
-typedef void CURLM;
+typedef struct Curl_multi CURLM;
 
 typedef enum {
   CURLM_CALL_MULTI_PERFORM = -1, /* please call curl_multi_perform() or