Stop attempting to make session logs private on Unix. This was introduced in

r7084 at the same time as sensible permissions when writing private key files;
however, it causes an assertion failure whenever an attempt is made to append
to an existing log file on Unix, and it's not clear what "is_private" *should*
do for append, so revert to log file security being the user's responsibility.
(Fixes Ubuntu LP#212711.)

git-svn-id: svn://svn.tartarus.org/sgt/putty@8461 cda61777-01e9-0310-a592-d414129be87e