USING '..' ON WINDOWS

Backport of the fix to 5.0 (to be null-merged to 5.1).
Moved the test into the main test suite. 
Made mysql-test-run.pl to not use symlinks for sdtdata as the symlinks
are now properly recognized by secure_file_priv.
Made sure the paths in load_file(), LOAD DATA and SELECT .. INTO OUTFILE 
that are checked against secure_file_priv in a correct way similarly to 5.1 
by the extended is_secure_file_path() backport before the comparison.
Added an extensive test with all the variants of upper/lower case, 
slash/backslash and case sensitivity.
Added few comments to the code.