Fix clang on Windows warnings

- STDCALL is ignored for variable argument functions.
- __attribute__ does work for Clang (also if that pretends to be MSVC)
- remove unused function
- simplify ma_getopt, fixes some strange compile error in clang (about
SSE intrinsics)

- fix some clang warnings

include/mysql/client_plugin.h

@@ -176,7 +176,7 @@ typedef struct st_mysql_client_plugin_REMOTEIO
   @retval
   a pointer to the loaded plugin, or NULL in case of a failure
 */
-struct st_mysql_client_plugin * STDCALL
+struct st_mysql_client_plugin *
 mysql_load_plugin(struct st_mysql *mysql, const char *name, int type,
                   int argc, ...);