[client] Code cleaning in net pkg and fix exit node feature on Android(#2932)

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: [struct_name] _ [functions] _ [os].go
- code cleaning in net_linux.go
- move env variables to env.go file

client/internal/routemanager/systemops/systemops_linux.go

@@ -55,7 +55,7 @@ type ruleParams struct {
 
 // isLegacy determines whether to use the legacy routing setup
 func isLegacy() bool {
-	return os.Getenv("NB_USE_LEGACY_ROUTING") == "true" || nbnet.CustomRoutingDisabled() || os.Getenv(nbnet.EnvSkipSocketMark) == "true"
+	return os.Getenv("NB_USE_LEGACY_ROUTING") == "true" || nbnet.CustomRoutingDisabled() || nbnet.SkipSocketMark()
 }
 
 // setIsLegacy sets the legacy routing setup