[krbdev.mit.edu #3498] race opening/creating replay cache.

[Ken Raeburn via RT]

To reproduce the problem, I had to add:

  0) Create an invalid replay cache, e.g., "echo hello >
/var/tmp/race_7882".

This is what causes the first invocation of the program to unlink the
file rather than recover data from it.

With some patches I just checked in (rev 18102), the error message
should be a little less useless, if you call krb5_get_error_message:

krb5_get_server_rcache(ctx, &piece, &rcache): Cannot create replay
cache: File exists

This does nothing about the actual race condition itself, of course.