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/constants.go

@@ -0,0 +1,16 @@
+package os
+
+import (
+	nativeos "os"
+)
+
+// Constants used for convenience so "os" does not have to be imported
+
+//nolint:golint
+const (
+	O_CREATE = nativeos.O_CREATE
+	O_TRUNC  = nativeos.O_TRUNC
+	O_WRONLY = nativeos.O_WRONLY
+	O_RDONLY = nativeos.O_RDONLY
+	O_RDWR   = nativeos.O_RDWR
+)