Martin Stein d5b1d9466a nic_router: clear ARP cache when domain is down
Whenever a domain looses all its interfaces or the link state of all attached
interfaces is down at once, the domain potentially moves to another Ethernet
segment and should therefore consider its ARP cache to be outdated.

RFC 826 states that "... If a host moves, any connections initiated by that
host will work, assuming its own address resolution table is cleared when it
moves. ...".

Therefore, this commit introduces clearing the ARP cache and the initially
stated events.

This commit was motivated by an issue with the PinePhone Modem and USB NIC.
On the PinePhone, the Modem has its own OS and acts as direct gateway to the
outer world for the USB NIC that is driven by Genode. However, whenever the
Modem gets restarted, Modem and USB NIC receive a new MAC address. This used
to conflict with the NIC routers ARP entry for the Modem that didn't cease to
be valid.

With this commit, the integrator of such a scenario at least has a convenient
way of fixing this by ensuring that all interfaces at the USB NIC domain go
down when resetting (e.g. by ensuring that the USB NIC is the only interface at
that domain).

Fixes #4558
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                      ===============================
                      Genode source-code repositories
                      ===============================


This directory contains the source-code repositories of the Genode OS
Framework. Each sub directory has the same principle layout as described in the
build-system manual:

:Build-system manual:

  [https://genode.org/documentation/developer-resources/build_system]

The build system uses a configurable selection of those reposities to obtain
the source codes for the build process. The repositories are not independent
but build upon of each other:

:'base':

  This directory contains the source-code repository of the fundamental
  frameworks and interfaces of Genode. Furthermore, it contains the generic
  parts of core.

:'base-<platform>':
  These directories contain platform-specific source-code repositories
  complementing the 'base' repository. The following platforms are supported:

  :'hw':
    The hw platform hosts Genode on a custom microkernel specifically
    developed for Genode. The name "hw" denotes that Genode is executed on
    bare hardware without a 3rd-party kernel underneath.

  :'linux':
    Linux kernel (both x86_32 and x86_64)

  :'nova':
    NOVA hypervisor ([https://hypervisor.org])

  :'foc':
    Fiasco.OC is a modernized version of the Fiasco microkernel with a
    completely revised kernel interface fostering capability-based
    security. It is not compatible with L4/Fiasco.

  :'okl4':
    OKL4 kernel (x86_32 and ARM) developed at Open-Kernel-Labs.

  :'pistachio':
    L4ka::Pistachio kernel developed at University of Karlsruhe.

  :'fiasco':
    L4/Fiasco kernel developed at University of Technology Dresden.

  :'sel4':
    seL4 microkernel ([https://sel4.systems/])

:'os':

  This directory contains the non-base OS components such as the init
  component, device drivers, and basic system services.

:'demo':

  This directory contains the source-code repository of various services and
  applications that we use for demonstration purposes. For example, a graphical
  application launcher called Launchpad and the Scout tutorial browser.

:'hello_tutorial':

  Tutorial for creating a simple client-server scenario with Genode. This
  repository includes documentation and the complete source code.

:'libports':

  This source-code repository contains ports of popular open-source libraries
  to Genode, most importantly the C library. The repository contains no
  upstream source code but means to download the code and adapt it to Genode.
  For instructions about how to use this mechanism, please consult the README
  file at the top level of the repository. Among the 3rd-party libraries
  are Qt5, freetype, ncurses, and Mesa.

:'dde_linux':

  This source-code repository contains the device driver environment for
  executing Linux subsystems as Genode components.

:'dde_ipxe':

  This source-code repository contains the device-driver environment for
  executing network drivers of the iPXE project.

:'dde_bsd':

  This source-code repository contains the device-driver environment for
  drivers of the OpenBSD operating system.

:'dde_rump':

  This source-code repository contains the port of rump kernels, which are
  used to execute subsystems of the NetBSD kernel as user level processes.
  The repository contains a server that uses a rump kernel to provide
  various NetBSD file systems to Genode.

:'pc':
  This source-code repository hosts device drivers that are specific for PC
  platforms. It depends on the 'dde_linux' repository.

:'ports':

  This source-code repository hosts ports of 3rd-party applications to
  Genode. The repository does not contain upstream source code but provides
  a mechanism for downloading the official source distributions and adapt
  them to the Genode environment. The used mechanism is roughly the same
  as used for the 'libports' repository. Please consult 'libports/README'
  for further information.

:'gems':

  This source-code repository contains Genode applications that use
  both native Genode interfaces as well as features of other high-level
  repositories, in particular shared libraries provided by 'libports'.