Refactor script (#92)

- cleaned some minor code
- use IPv6 if IPv4 isn't available
- add input validations, fixes #86 .
  - assign secondary DNS to primary DNS value if it's empty. Fixes #68 
- use `$SERVER_WG_IPV4` and `$SERVER_WG_IPV6` when suggesting the client IP
- save user DNS to `params` file and apply it to all clients, removing support for customizing DNS for individual clients. If this is a problem we can add again the code to allow customization but maybe use the sourced DNS as a suggestion for clients that need a different one
- add shellcheck ignores, needed for IDE that have shellcheck support
- escaped variables to `"${var}"` style
- updated README to reflect changes
- fixed the kernel mismatch issue on CentOS by updating the kernel #95 (comment). Fedora might need this change too
- Use `firewall-cmd` only if `firewalld` is running. Fixes #95 
- Fix the client's subnet mask. Fixes #87 
- Save the client configuration in the correct `$HOME`. Fixes #96 

Co-authored-by: Chris Lewicki <chris@lewicki.dev>
Co-authored-by: Stanislas <angristan@pm.me>

README.md

@@ -37,12 +37,6 @@ chmod +x wireguard-install.sh
 
 It will install WireGuard (kernel module and tools) on the server, configure it, create a systemd service and a client configuration file.
 
-To generate more client files, run the following:
-
-```sh
-./wireguard-install.sh add-client
-```
-
 Make sure you choose different IPs for you clients.
 
 Contributions are welcome!