Fix `puttygen-unix-perms': f_open(), PuTTY's wrapper on fopen, now

Fix `puttygen-unix-perms': f_open(), PuTTY's wrapper on fopen, now
takes a third argument which is TRUE if the file is being opened for
writing and wants to be created in such a way that it's readable
only to the owner. This is used when saving private keys.

While I'm here, I also use this option when writing session logs, on
the general principle that they probably contain _something_
sensitive.

The new argument is only supported on Unix, for the moment. (I think
writing owner-accessible-only files is the default on Windows.)

[originally from svn r7084]