MDEV-11170: MariaDB 10.2 cannot start on MySQL 5.7 datadir

PART 1 of the fix requires a bit of refactoring to not use hard-coded
field indices any more. Create classes that express the grant tables structure,
without exposing the underlying field indices.

Most of the code is converted to use these classes, except parts which
are not directly affected by the MDEV-11170. These however are TODO
items for subsequent refactoring.

sql/structs.h

@@ -205,7 +205,7 @@ typedef int *(*update_var)(THD *, struct st_mysql_show_var *);
 typedef struct	st_lex_user {
   LEX_STRING user, host, plugin, auth;
   LEX_STRING pwtext, pwhash;
-  bool is_role() { return user.str[0] && !host.str[0]; }
+  bool is_role() const { return user.str[0] && !host.str[0]; }
   void set_lex_string(LEX_STRING *l, char *buf)
   {
     if (is_role())