[NET]: Add network namespace clone & unshare support.

This patch allows you to create a new network namespace
using sys_clone, or sys_unshare.

As the network namespace is still experimental and under development
clone and unshare support is only made available when CONFIG_NET_NS is
selected at compile time.

As this patch introduces network namespace support into code paths
that exist when the CONFIG_NET is not selected there are a few
additions made to net_namespace.h to allow a few more functions
to be used when the networking stack is not compiled in.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/Kconfig

@@ -27,6 +27,14 @@ if NET
 
 menu "Networking options"
 
+config NET_NS
+	bool "Network namespace support"
+	default n
+	depends on EXPERIMENTAL && !SYSFS
+	help
+	  Allow user space to create what appear to be multiple instances
+	  of the network stack.
+
 source "net/packet/Kconfig"
 source "net/unix/Kconfig"
 source "net/xfrm/Kconfig"