ARP never seems to remove rejected entries

[axeld]

When I boot my main system, I often cannot access my NAS. And that has always the same reason: there is an ARP "reject" entry for that host.

There are two issues at play here, that should be fixed:

• rejected entries should be removed after 20 seconds, but apparently, that never happens.
• when the stack receives a packet from the rejected host, it should remove the reject entry automatically, as long as it is not permanent.