OpenVPN: Added additional configuration for server and clients

OpenVPN: Added additional configuration for server and clients

* Added a possibility to manualy extend OpenVPNs server and client configuration.
* Added also a checkbox (on/off) in the WUI under 'Advanced server options' .
* Changed the order in 'Miscellaneous options' section for better overview.
* Optimized code in particular sections a little.
    Added a filehandle instead of system(touch...) for ccd* file generation.
    Unified the html code tags in processed section.
Fixes #10577