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]

windows/winstuff.h

@@ -18,7 +18,7 @@
 struct Filename {
     char path[FILENAME_MAX];
 };
-#define f_open(filename, mode) ( fopen((filename).path, (mode)) )
+#define f_open(filename, mode, isprivate) ( fopen((filename).path, (mode)) )
 
 struct FontSpec {
     char name[64];