[krbdev.mit.edu #3499] race in replay cache file ownership

[Ken Raeburn via RT]

We should do the check after opening.

However, there are device files on some UNIX platforms where opening the file at all can have 
potentially undesirable effects.  So I think it's probably a good idea to keep the check before 
opening, as well.  (Though perhaps we want to use lstat, and make sure the replay cache "file" 
isn't actually a symlink.)