authd: wait until the ssl connection is "open" before reading

authd: wait until the ssl connection is "open" before reading

It's useful to allow authd to run in parallel with ssl negotiation,
but if the ssld connection has plaintext data ready for reading
there's a race condition between authd calling read_packet() and
ssl_process_certfp() storing the certificate fingerprint. This
scenario would be bad for a server connecting because fingerprint
verification will fail.

Allow either operation to complete first, but wait until
ssl_process_open_fd() calls the ssl open callback before calling
read_packet().