[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

util/net/net_linux.go

@@ -4,29 +4,42 @@ package net
 
 import (
 	"fmt"
-	"os"
 	"syscall"
 
 	log "github.com/sirupsen/logrus"
 )
 
-const EnvSkipSocketMark = "NB_SKIP_SOCKET_MARK"
-
 // SetSocketMark sets the SO_MARK option on the given socket connection
 func SetSocketMark(conn syscall.Conn) error {
+	if isSocketMarkDisabled() {
+		return nil
+	}
+
 	sysconn, err := conn.SyscallConn()
 	if err != nil {
 		return fmt.Errorf("get raw conn: %w", err)
 	}
 
-	return SetRawSocketMark(sysconn)
+	return setRawSocketMark(sysconn)
 }
 
-func SetRawSocketMark(conn syscall.RawConn) error {
+// SetSocketOpt sets the SO_MARK option on the given file descriptor
+func SetSocketOpt(fd int) error {
+	if isSocketMarkDisabled() {
+		return nil
+	}
+
+	return setSocketOptInt(fd)
+}
+
+func setRawSocketMark(conn syscall.RawConn) error {
 	var setErr error
 
 	err := conn.Control(func(fd uintptr) {
-		setErr = SetSocketOpt(int(fd))
+		if isSocketMarkDisabled() {
+			return
+		}
+		setErr = setSocketOptInt(int(fd))
 	})
 	if err != nil {
 		return fmt.Errorf("control: %w", err)
@@ -39,17 +52,18 @@ func SetRawSocketMark(conn syscall.RawConn) error {
 	return nil
 }
 
-func SetSocketOpt(fd int) error {
-	if CustomRoutingDisabled() {
-		log.Infof("Custom routing is disabled, skipping SO_MARK")
-		return nil
-	}
-
-	// Check for the new environment variable
-	if skipSocketMark := os.Getenv(EnvSkipSocketMark); skipSocketMark == "true" {
-		log.Info("NB_SKIP_SOCKET_MARK is set to true, skipping SO_MARK")
-		return nil
-	}
-
+func setSocketOptInt(fd int) error {
 	return syscall.SetsockoptInt(fd, syscall.SOL_SOCKET, syscall.SO_MARK, NetbirdFwmark)
 }
+
+func isSocketMarkDisabled() bool {
+	if CustomRoutingDisabled() {
+		log.Infof("Custom routing is disabled, skipping SO_MARK")
+		return true
+	}
+
+	if SkipSocketMark() {
+		return true
+	}
+	return false
+}