10.2 integration fixes

- changed plugin API to avoid crashes: Oracle/MariaDB changed
    structure several times without updating interface version.
  - ABI fixes: moved additional net items to net->extension (connection
    handler and com_multi buffer)

include/mysql.h

@@ -424,8 +424,10 @@ typedef struct character_set
   const char *desc;                                     \
   unsigned int version[3];                              \
   const char *license;                                  \
+  void *mariadb_api;                                    \
   int (*init)(char *, size_t, int, va_list);            \
-  int (*deinit)();
+  int (*deinit)();                                      \
+  int (*options)(const char *option, const void *);
 struct st_mysql_client_plugin
 {
   MYSQL_CLIENT_PLUGIN_HEADER