refactor: consolidate network interface detection logic

Move platform-specific network interface detection from cmd/cli/ to root package
as ValidInterfaces function. This eliminates code duplication and provides a
consistent interface for determining valid physical network interfaces across
all platforms.

- Remove duplicate validInterfacesMap functions from platform-specific files
- Add context parameter to virtualInterfaces for proper logging
- Update all callers to use ctrld.ValidInterfaces instead of local functions
- Improve error handling in virtual interface detection on Linux

nameservers_linux_test.go

@@ -1,10 +1,11 @@
 package ctrld
 
 import (
+	"context"
 	"testing"
 )
 
 func Test_virtualInterfaces(t *testing.T) {
-	vis := virtualInterfaces()
+	vis := virtualInterfaces(context.Background())
 	t.Log(vis)
 }