feat(ipv6): Add full IPv6-only mode with manual hostname/IP fallback …

[Knogle]

…if no IPv4

• Detects absence of global IPv4 and prompts user for manual public IPv4 or hostname.
• If user declines, falls back to checking for public IPv6 address.
• If global IPv6 is available, offers IPv6-only mode as a valid setup path.
• Ensures $IP and $ENDPOINT are set appropriately in all scenarios.
• Updates public IP fallback logic to also handle IPv6-only configurations.
• Adds support for manual entry of public IPv6 hostname/IP when behind NAT or no IPv4 is available.

This improves support for IPv6-only server deployments and ensures usability in NAT64, DS-Lite, or cloud environments without native IPv4.

root@OVPN-Test:~# ./openvpn-install.sh 
Welcome to the OpenVPN installer!
The git repository is available at: https://github.com/angristan/openvpn-install

I need to ask you a few questions before starting the setup.
You can leave the default options and just press enter if you are OK with them.

IP address for OpenVPN to listen on: 2a00:XXXX:XXXX:XXXX:XXX:XXXX:XXXX:XXXX

Checking for IPv6 connectivity...

Your host appears to have IPv6 connectivity.

Do you want to enable IPv6 support (NAT)? [y/n]: y

What port do you want OpenVPN to listen to?
   1) Default: 1194
   2) Custom
   3) Random [49152-65535]
Port choice [1-3]: 1