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

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.)

[originally from svn r8461]
[r7084 == 4fa9564c909c589bcccc95d57fae5469063c1759]