MDEV-5215 post-review fixes

* "public" should work in any letter case
* PUBLIC is not a valid definer
* granting to public should auto-create an entry in mysql.global_priv
* SHOW GRANTS should show privileges obtained via PUBLIC
* LEX_USER::is_public was often uninitialized
* comments, whitespaces, typos, etc

sql/sql_class.cc

@@ -6033,6 +6033,7 @@ void THD::get_definer(LEX_USER *definer, bool role)
     definer->user= invoker.user;
     definer->host= invoker.host;
     definer->auth= NULL;
+    definer->is_public= false;
   }
   else
 #endif