Add check for bind-dev in DCO options

[BCurrell]

Thank you for your contribution

You are welcome to open PR, but they are used for discussion only. All
patches must eventually go to the openvpn-devel mailing list for review:

• https://lists.sourceforge.net/lists/listinfo/openvpn-devel

Please send your patch using git-send-email. For example to send your latest commit to the list:

$ git send-email [email protected] HEAD~1

For details, see these Wiki articles:

• https://community.openvpn.net/openvpn/wiki/DeveloperDocumentation
• https://community.openvpn.net/openvpn/wiki/Contributing

[BCurrell]

Following from #683, add an option to check for the bind-dev option and disable DCO. I'm not sure if this is the correct function to add it to.

Also, isn't bind-dev specific to Linux, so do I need to add it to an if block checking for Linux?

[cron2]

@ordex I don't think we should disable DCO in this case. We should fix it inside DCO, not weasel out :-)

[ordex]

@ordex I don't think we should disable DCO in this case. We should fix it inside DCO, not weasel out :-)

@cron2 that's true, but for the time being it may be easier to let people know that this option is not supported.
Adding support to DCO (ovpn-dco or ovpn? ovpn-dco is in maintenance mode right now) may requires till some time.