Maint: split custom config files in openvpn/custom

internal/openvpn/custom/read.go

@@ -0,0 +1,27 @@
+package custom
+
+import (
+	"io"
+	"os"
+	"strings"
+)
+
+func readCustomConfigLines(filepath string) (
+	lines []string, err error) {
+	file, err := os.Open(filepath)
+	if err != nil {
+		return nil, err
+	}
+
+	b, err := io.ReadAll(file)
+	if err != nil {
+		_ = file.Close()
+		return nil, err
+	}
+
+	if err := file.Close(); err != nil {
+		return nil, err
+	}
+
+	return strings.Split(string(b), "\n"), nil
+}