[client] make /var/lib/netbird paths configurable (#3084)

- NB_STATE_DIR
- NB_UNCLEAN_SHUTDOWN_RESOLV_FILE
- NB_DNS_STATE_FILE

client/configs/configs.go

@@ -0,0 +1,24 @@
+package configs
+
+import (
+	"os"
+	"path/filepath"
+	"runtime"
+)
+
+var StateDir string
+
+func init() {
+	StateDir = os.Getenv("NB_STATE_DIR")
+	if StateDir != "" {
+		return
+	}
+	switch runtime.GOOS {
+	case "windows":
+		StateDir = filepath.Join(os.Getenv("PROGRAMDATA"), "Netbird")
+	case "darwin", "linux":
+		StateDir = "/var/lib/netbird"
+	case "freebsd", "openbsd", "netbsd", "dragonfly":
+		StateDir = "/var/db/netbird"
+	}
+}