dco.c: check certain options only on startup

dco.c: check certain options only on startup

Following options are set on startup and cannot be changed later:

 - dev
 - dev-type
 - connections list
 - mode
 - topology

Same for system-wide availability of dco.

dco_check_option_conflict(), where those options
were checked, is also called in server mode when
client is connected. Move those checks to
dco_check_startup_option_conflict() which is only
called at startup.

Since we moved dco_enabled() check to startup,
dco_check_option_conflict() might now trigger exit
on Windows if system lacks chachapoly support.
Since dco checks only need to be performed for
dco, wrap those into "if (dco_enabled) {}".

Signed-off-by: Lev Stipakov <lev@openvpn.net>
Acked-by: Antonio Quartulli <a@unstable.cc>
Message-Id: <20220909121841.646-1-lstipakov@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25158.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>