Fix initialisation of packet decryption context broken by commit 3308d13e7e3bf20cfeaf...

Fix initialisation of packet decryption context broken by commit 3308d13e7e3bf20cfeaf6f2ab17228a9820cea66.

Instead of a single, global decryption context, each node has its own context.
However, in send_ans_key(), the global context was initialised. This commit
fixes that and removes the global context completely.

Also only set status.validkey after all checks have been evaluated.