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_emem.c

@@ -189,6 +189,7 @@ krb5_storage_emem(void)
     sp->store = emem_store;
     sp->seek = emem_seek;
     sp->trunc = emem_trunc;
+    sp->fsync = NULL;
     sp->free = emem_free;
     sp->max_alloc = UINT_MAX/8;
     return sp;