Iterative patch improvement. Previously committed patch
caused wrong result on Windows. The previous patch also
broke secure_file_priv for symlinks since not all file
paths which must be compared against this variable are
normalized using the same norm.

The server variable opt_secure_file_priv wasn't
normalized properly and caused the operations
LOAD DATA INFILE .. INTO TABLE ..
and
SELECT load_file(..)
to do different interpretations of the 
--secure-file-priv option.
     
The patch moves code to the server initialization
routines so that the path always is normalized
once and only once.
      
It was also intended that setting the option
to an empty string should be equal to 
lifting all previously set restrictions. This
is also fixed by this patch.


mysql-test/r/loaddata.result:
  * Removed test code which will currently break the much used --mem feature of mtr.
mysql-test/t/loaddata.test:
  * Removed test code which will currently break the much used --mem feature of mtr.
sql/item_strfunc.cc:
  * Replaced string comparing code on opt_secure_file_priv with an interface which guarantees that both file paths are normalized using the same norm on all platforms.
sql/mysql_priv.h:
  * Added signature for is_secure_file_path()
sql/mysqld.cc:
  * New function for checking if a path compatible with the secure path restriction.
  * Added initialization of the opt_secure_file_priv variable.
sql/sql_class.cc:
  * Replaced string comparing code on opt_secure_file_priv with an interface which guarantees that both file paths are normalized using the same norm on all platforms.
sql/sql_load.cc:
  * Replaced string comparing code on opt_secure_file_priv with an interface which guarantees that both file paths are normalized using the same norm on all platforms.