test_if_data_home_dir fixed to look into real path.
      Checks added to mi_open for symlinks into data home directory.
modified:
  include/my_sys.h
  include/myisam.h
  myisam/mi_check.c
  myisam/mi_open.c
  myisam/mi_static.c
  myisam/myisamchk.c
  myisam/myisamdef.h
  mysql-test/r/symlink.result
  mysys/my_symlink.c
  sql/mysql_priv.h
  sql/mysqld.cc
  sql/sql_parse.cc

per-file messages:
  include/my_sys.h
    Bug#32167 another privilege bypass with DATA/INDEX DIRECTORY.
    
    my_is_symlink interface added
  include/myisam.h
    Bug#32167 another privilege bypass with DATA/INDEX DIRECTORY.
    
    myisam_test_invalid_symlink interface added
  myisam/mi_check.c
    Bug#32167 another privilege bypass with DATA/INDEX DIRECTORY.
    
    mi_open_datafile calls modified
  myisam/mi_open.c
    Bug#32167 another privilege bypass with DATA/INDEX DIRECTORY.
    
    code added to mi_open to check for symlinks into data home directory.
    mi_open_datafile now accepts 'original' file path to check if it's
    an allowed symlink.
  myisam/mi_static.c
    Bug#32167 another privilege bypass with DATA/INDEX DIRECTORY.
    
    myisam_test_invlaid_symlink defined
  myisam/myisamchk.c
    Bug#32167 another privilege bypass with DATA/INDEX DIRECTORY.
    
    mi_open_datafile call modified
  myisam/myisamdef.h
    Bug#32167 another privilege bypass with DATA/INDEX DIRECTORY.
    
    mi_open_datafile interface modified - 'real_path' parameter added
  mysql-test/r/symlink.test
    Bug#32167 another privilege bypass with DATA/INDEX DIRECTORY.
    
    error codes corrected as some patch now rejected pointing inside datahome
  mysql-test/r/symlink.result
    Bug#32167 another privilege bypass with DATA/INDEX DIRECTORY.
    
    error messages corrected in the result
  mysys/my_symlink.c
    Bug#32167 another privilege bypass with DATA/INDEX DIRECTORY.
    
    my_is_symlink() implementsd
    my_realpath() now returns the 'realpath' even if a file isn't a symlink
  sql/mysql_priv.h
    Bug#32167 another privilege bypass with DATA/INDEX DIRECTORY.
    
    test_if_data_home_dir interface
  sql/mysqld.cc
    Bug#32167 another privilege bypass with DATA/INDEX DIRECTORY.
    
    myisam_test_invalid_symlik set with the 'test_if_data_home_dir'
  sql/sql_parse.cc
    Bug#32167 another privilege bypass with DATA/INDEX DIRECTORY.
    
    error messages corrected
    test_if_data_home_dir code fixed