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/funcs.go

@@ -0,0 +1,10 @@
+package os
+
+import (
+	nativeos "os"
+)
+
+type OpenFileFunc func(name string, flag int, perm FileMode) (File, error)
+type MkdirAllFunc func(name string, perm nativeos.FileMode) error
+type RemoveFunc func(name string) error
+type ChownFunc func(name string, uid int, gid int) error