Fix compile issue and add -force flag to bypass VPN check

- Updated golang.org/x/net dependency to fix linking error with syscall.recvmsg
- Added -force flag to allow running the tool while connected to VPN
- Fixed linter errors: replaced deprecated ioutil with io/os, added error handling
- Added test, fmt, and lint targets to Makefile

Makefile

@@ -5,3 +5,12 @@ build:
 
 run:
 	sudo go run ./cmd/mullvadclosest/*.go
+
+test:
+	go test -v ./...
+
+fmt:
+	go fmt ./...
+
+lint:
+	golangci-lint run