LibC: Implement popen() and pclose().

I feel reasonably confident that I might have gotten these right. :^)

AK/ValueRestorer.h

@@ -0,0 +1,26 @@
+#pragma once
+
+namespace AK {
+
+template<typename T>
+class ValueRestorer {
+public:
+    ValueRestorer(T& variable)
+        : m_variable(variable)
+        , m_saved_value(variable)
+    {
+    }
+
+    ~ValueRestorer()
+    {
+        m_variable = m_saved_value;
+    }
+
+private:
+    T& m_variable;
+    T m_saved_value;
+};
+
+}
+
+using AK::ValueRestorer;