LibCore: Make get_current_user_home_path() return String & close passwd

This API was returning a "const char*" and it was unclear who took care
of the underlying memory. Returning a String makes that obvious.

Also make sure we close the /etc/passwd file when we're done with it.

Libraries/LibCore/CUserInfo.h

@@ -1 +1,5 @@
-const char* get_current_user_home_path();
+#pragma once
+
+#include <AK/AKString.h>
+
+String get_current_user_home_path();