Code maintenance: OS package for file system

- OS custom internal package for file system interaction
- Remove fileManager external dependency
- Closer API to Go's native API on the OS
- Create directories at startup
- Better testability
- Move Unsetenv to os interface

internal/provider/purevpn.go

@@ -10,7 +10,7 @@ import (
 	"github.com/qdm12/gluetun/internal/constants"
 	"github.com/qdm12/gluetun/internal/firewall"
 	"github.com/qdm12/gluetun/internal/models"
-	"github.com/qdm12/golibs/files"
+	"github.com/qdm12/gluetun/internal/os"
 	"github.com/qdm12/golibs/logging"
 )
 
@@ -150,7 +150,7 @@ func (p *purevpn) BuildConf(connection models.OpenVPNConnection, verbosity int,
 }
 
 func (p *purevpn) PortForward(ctx context.Context, client *http.Client,
-	fileManager files.FileManager, pfLogger logging.Logger, gateway net.IP, fw firewall.Configurator,
+	openFile os.OpenFileFunc, pfLogger logging.Logger, gateway net.IP, fw firewall.Configurator,
 	syncState func(port uint16) (pfFilepath models.Filepath)) {
 	panic("port forwarding is not supported for purevpn")
 }