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

langs/de/cgi-bin/de.pl

@@ -1689,6 +1689,7 @@
 'outgoing traffic in bytes per second' => 'Abgehender Verkehr',
 'override mtu' => 'Überschreibe Standard MTU',
 'ovpn' => 'OpenVPN',
+'ovpn add conf' => 'Erweiterte Konfiguration',
 'ovpn con stat' => 'OpenVPN Verbindungs-Statistik',
 'ovpn config' => 'OVPN-Konfiguration',
 'ovpn crypt options' => 'Kryptografieoptionen',

langs/en/cgi-bin/en.pl

@@ -1720,6 +1720,7 @@
 'outgoing traffic in bytes per second' => 'Outgoing Traffic',
 'override mtu' => 'Override default MTU',
 'ovpn' => 'OpenVPN',
+'ovpn add conf' => 'Additional configuration',
 'ovpn con stat' => 'OpenVPN Connection Statistics',
 'ovpn config' => 'OVPN-Config',
 'ovpn crypt options' => 'Cryptographic options',