added new function test_if_data_home_dir() which checks that
path does not contain mysql data home directory.
Using of mysql data home directory in
DATA DIRECTORY & INDEX DIRECTORY is disallowed.


mysql-test/r/symlink.result:
  test result
mysql-test/t/symlink.test:
  test case
sql/mysql_priv.h:
  new variable mysql_unpacked_real_data_home
sql/mysqld.cc:
  new variable mysql_unpacked_real_data_home
sql/sql_parse.cc:
  added new function test_if_data_home_dir() which checks that
  path does not contain mysql data home directory.
  Using of mysql data home directory in
  DATA DIRECTORY & INDEX DIRECTORY is disallowed.