[krbdev.mit.edu #2763] file ccache should be held open while scanning for credentials

[Ken Raeburn via RT]

Fixing this is going to require extending the ccache operations table, 
because while there is a set_flags operation, there is no get_flags 
operation, so saving and restoring the old flags can't be done under 
the current interface.  Fortunately, the struct is internal, and we'd 
just be adding a new function.