PREPARE and EXECUTE of statement breaks binlog.


sql/sp.cc:
  - Polishing sp_use_new_db():
    - renamed no_access_check to force_switch to be more adequate;
    - fixed comment;
sql/sql_class.h:
  Polishing: fixed comment.
sql/sql_db.cc:
  1. Use mysql_change_db_impl() to reset current database instead of
  THD::set_db() in mysql_rm_db(). THD::set_db() does not take care of
  THD::db_access and database attributes (@@collation_database).
  2. Polishing: add, fix comments.