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/os/file.go

@@ -0,0 +1,15 @@
+package os
+
+import (
+	"io"
+	nativeos "os"
+)
+
+//go:generate mockgen -destination=mock_$GOPACKAGE/$GOFILE . File
+
+type File interface {
+	io.ReadWriteCloser
+	WriteString(s string) (int, error)
+	Chown(uid, gid int) error
+	Chmod(mode nativeos.FileMode) error
+}