Implement exit notification via control channel

Implement exit notification via control channel

Current exit notification relies on data channel messages with specific
prefix. Adding these to new data channel modules (DCO) adds unncessary
complexity for the data for messages that from their idea belong to the
control channel anyway.

This patch adds announcing support for control channel and sending/receving
it. We use the simple EXIT message for this.

Patch v2: add comment about protocol-flags to be not a user visible option,
          fix various grammar mistakes, remove unused argument to
          receive_exit_message

Patch v3: rename data_channel_crypto_flags to imported_protocol_flags
          add tls-ekm to protocol-flags.

Patch v4: rebase, use a buffer for the code that prepares the push reply

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Heiko Hund <heiko@ist.eigentlich.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20220914165041.2658423-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25209.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>