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]