Extract check_session_cipher into standalone function

Extract check_session_cipher into standalone function

This allow the code later to check if the cipher is okay to use and
update it for the calculation for the max MTU size.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Patch v2: Name function check_session_cipher to better reflect its
          function
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20220729123748.3267207-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg24766.html

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