[client] Code cleaning in net pkg

[pappz]

Describe your changes

Code cleaning around the util/net package. The goal was to write a more understandable source code but modify nothing on the logic. Protect the WireGuard UDP listeners with marks. The implementation can support the VPN permission revocation events in thread safe way. It will be important if we start to support the running time route and DNS update features.

• uniformize the file name convention: [structname] [functions] _ [os].go
• code cleaning in net_linux.go
• move env variables to env.go file

Issue ticket number and link

Checklist

• [x] Is it a bug fix
• [ ] Is a typo/documentation fix
• [ ] Is a feature enhancement
• [x] It is a refactor
• [ ] Created tests that fail without the change (if possible)
• [ ] Extended the README / documentation, if necessary

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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