Use ephemeral X.509 credentials if none are configured

If kresd is configured to listen using TLS, but it has no credentials,
it should fall back to generating ephemeral credentials and using
them.

It stores the ephemerally-generated secret key in the same directory
as the cache, using the name "ephemeral_key.pem".  If the cache
persists, then the key will too, even if the daemon dies.  This means
that any set of daemons that share a cache will also share an
ephemeral secret key.

The ephemeral X.509 certificate that corresponds to the key will be
automatically generated (self-signed), will have a lifetime of about
90 days (matching Let's Encrypt policy).  The ephemeral cert is
never written to disk; it is always dynamically-generated by kresd.

This should make it very easy to get DNS-over-TLS working in
opportunistic mode.