s_client: Support interactive reconnect command

s_client: Support interactive reconnect command

This change adds the 'C' command character to allow a user to drop the
current session and reconnect to the server. It has the same behavior as
the `-reconnect` option except this allows reconnect to be triggered at
an arbitrary point in the session.

The primary use case for this change is to provide a way for the user to
probe a server for TLS1.3 session resumption support. This is not
currently reliably supported by the `-reconnect` option.

Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/18683)