GNetworkMonitorNetlink: Fix check for non-kernel messages

This code used to look at the SCM_CREDENTIALS and ignore every message
not from uid 0. However, when user namespaces are in use this does not
work, as if uid 0 is not mapped you get overflowuid instead. Right now
this means we ignore all messages in such user namespaces and glib
apps hang on startup.

We can't look at pids either, as pid 0 is returned for processes
outside your pid namespace.

Instead the correct approach is to look at the sending sockaddr and
if the port id (nl_pid) is zero, then its from the kernel.

Source:
http://lists.linuxfoundation.org/pipermail/containers/2015-May/036032.html

https://bugzilla.gnome.org/show_bug.cgi?id=750203