chore(all): return concrete types, accept interfaces

- Remove exported interfaces unused locally
- Define interfaces to accept arguments
- Return concrete types, not interfaces

internal/openvpn/config.go

@@ -5,10 +5,6 @@ import (
 	"strings"
 )
 
-type Writer interface {
-	WriteConfig(lines []string) error
-}
-
 func (c *Configurator) WriteConfig(lines []string) error {
 	file, err := os.OpenFile(c.configPath, os.O_CREATE|os.O_TRUNC|os.O_WRONLY, 0644)
 	if err != nil {