Move tls_process_state into its own function

This function does most of the state transitions in the TLS state
machine. Moving it into its own function removes an intention area and
makes tls_process function easier to understand as the loop is more
obvious.

This is largely just a code move with small expection. bool active is
no longer directly set but inferred from to_link->len

Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Message-Id: <20220422142953.3805364-5-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg24157.html

Signed-off-by: Gert Doering <gert@greenie.muc.de>