include: revert 9adf3c4 and make public types void * again

Many applications assume the actual contents of the public types and use
that do for example forward declarations (saving them from including our
public header) which then breaks when we switch from void * to a struct
*.

I'm not convinced we were wrong, but since this practise seems
widespread enough I'm willing to (partly) step down.

Now libcurl uses the struct itself when it is built and it allows
applications to use the struct type if CURL_STRICTER is defined at the
time of the #include.

Reported-by: Peter Frühberger
Fixes #926

include/curl/curl.h

@@ -91,7 +91,13 @@
 extern "C" {
 #endif
 
+#if defined(BUILDING_LIBCURL) || defined(CURL_STRICTER)
 typedef struct Curl_easy CURL;
+typedef struct Curl_share CURLSH;
+#else
+typedef void CURL;
+typedef void CURLSH;
+#endif
 
 /*
  * libcurl external API function linkage decorations.
@@ -2258,7 +2264,6 @@ typedef void (*curl_unlock_function)(CURL *handle,
                                      curl_lock_data data,
                                      void *userptr);
 
-typedef struct Curl_share CURLSH;
 
 typedef enum {
   CURLSHE_OK,  /* all is fine */

include/curl/multi.h

@@ -52,7 +52,11 @@
 extern "C" {
 #endif
 
+#if defined(BUILDING_LIBCURL) || defined(CURL_STRICTER)
 typedef struct Curl_multi CURLM;
+#else
+typedef void CURLM;
+#endif
 
 typedef enum {
   CURLM_CALL_MULTI_PERFORM = -1, /* please call curl_multi_perform() or