Add krb5_storage_fsync().

We add a function to cause krb5_storage's to be sync'd to their backing
store.  For memory backed storages, this is a NOP.  For files, it calls
fsync on the file descriptor.

lib/krb5/store-int.h

@@ -40,6 +40,7 @@ struct krb5_storage_data {
     ssize_t (*store)(struct krb5_storage_data*, const void*, size_t);
     off_t (*seek)(struct krb5_storage_data*, off_t, int);
     int (*trunc)(struct krb5_storage_data*, off_t);
+    int (*fsync)(struct krb5_storage_data*);
     void (*free)(struct krb5_storage_data*);
     krb5_flags flags;
     int eof_code;