ulogd: add linux namespace helper

ulogd: add linux namespace helper

The new namespace helper provides an internal stable interface for
plugins to use for switching various linux namespaces. Currently only
network namespaces are supported/implemented, but can easily be extended
if needed. autoconf will enable it automatically if the required symbols
are available. If ulogd is compiled without namespace support, the
functions will simply return an error, there is no need for conditional
compilation or special handling in plugin code.

Signed-off-by: Corubba Smith <corubba@gmx.de>
Signed-off-by: Florian Westphal <fw@strlen.de>