[krbdev.mit.edu #2874] locking bug in file ccache

[Ken Raeburn via RT]

There seems to be a locking bug in cc_file.c where one thread,
changing the "OPENCLOSE" mode, wants to release disk_file_lock when
it's locked by a different thread.  This seems to show up on x86-linux
under valgrind with Jeff Altman's modified multithreaded gss sample
program, but not without valgrind.

My locking strategy overall in that file is poorly thought out and
needs some work (and documentation).  This should be fixed for 1.4.

Ken