Fix for bug #4815 (embedded server calculates wrong places for outfiles)

In some places in mysqld behaviour depends on system working directory
It works badly in libmysqld because user can set it in the way he needs.
I think we should explicitly insert mysql_real_data_home value in
paths in these places


sql/sql_class.cc:
  here we concat mysql_real_data_home and thd->db to be the prefix
sql/sql_load.cc:
  it's better to build the prefix from mysql_real_data_home
  also i think it's better always to call my_load_path to not to depend
  of current system working directory