Kernel: More work towards POSIX SHM, also add ftruncate().

Kernel/SharedMemory.h

@@ -0,0 +1,22 @@
+#pragma once
+
+#include <AK/Retainable.h>
+#include <AK/RetainPtr.h>
+#include <Kernel/KResult.h>
+
+class VMObject;
+
+class SharedMemory : public Retainable<SharedMemory> {
+public:
+    static Retained<SharedMemory> create();
+    ~SharedMemory();
+
+    KResult truncate(int);
+
+private:
+    SharedMemory();
+
+    int m_uid { 0 };
+    int m_gid { 0 };
+    RetainPtr<VMObject> m_vmo;
+};