refactor: move getDNS type to os_linux.go

Move getDNS type definition from dns.go to os_linux.go where it is used.
Remove the now-empty dns.go file. This change improves code organization
by keeping platform-specific types with their implementations.

cmd/cli/os_linux.go

@@ -28,6 +28,8 @@ import (
 
 const resolvConfBackupFailedMsg = "open /etc/resolv.pre-ctrld-backup.conf: read-only file system"
 
+type getDNS func(iface string) []string
+
 // allocate loopback ip
 // sudo ip a add 127.0.0.2/24 dev lo
 func allocateIP(ip string) error {