MINOR: ssl-sample: add ssl_fc_early_rcvd() to detect use of early data

MINOR: ssl-sample: add ssl_fc_early_rcvd() to detect use of early data

We currently have ssl_fc_has_early() which says that early data are still
unconfirmed by a final handshake, but nothing to see if a client has been
able to use early data at all, which is a problem because such mechanisms
generally depend on multiple factors and it's hard to know when they start
to work. This new sample fetch function will indicate that some early data
were seen over that front connection, i.e. this can be used to confirm
that at some point the client was able to push some. This is essentially
a debugging tool that has no practical use case other than debugging.