Direct Code Execution upgrade

Direct Code Execution is a framework for ns-3. It allows the users to execute Linux kernel and Linux userspace networking applications inside ns-3 without changing their code. In order to provide such functionality, DCE integrates network simulator with custom Linux kernels like net-next-sim or net-next-nuse (LibOs). But net-next-nuse has not kept pace with latest versions of Linux kernel. This has been the major hindrance for DCE to use latest Linux kernel. An active alternative to LibOs is Linux Kernel Library (LKL), which provides similar features of LibOs and supports latest Linux kernel versions. In this project, LibOs will be replaced with LKL from DCE to support the networking stack of Linux. Currently, DCE uses custom implementation of libc; this makes it hard to maintain the code base. The another aspect of this project is to drop the custom libc and embed the musl libc in DCE.