remove sys_var specific restore_pluginvar_names() function,

use generic restore_ptr_backup() approach

sql/sql_plugin.h

@@ -85,6 +85,7 @@ struct st_ptr_backup {
   void **ptr;
   void *value;
   void save(void **p) { ptr= p; value= *p; }
+  void save(const char **p) { save((void**)p); }
   void restore() { *ptr= value; }
 };
 
@@ -108,6 +109,8 @@ struct st_plugin_int
   LEX_STRING name;
   struct st_maria_plugin *plugin;
   struct st_plugin_dl *plugin_dl;
+  st_ptr_backup *ptr_backup;
+  uint nbackups;
   uint state;
   uint ref_count;               /* number of threads using the plugin */
   uint locks_total;             /* how many times the plugin was locked */