[NETFILTER]: add /proc/net/netfilter interface to nf_queue

This patch adds a /proc/net/netfilter/nf_queue file, similar to the
recently-added /proc/net/netfilter/nf_log.  It indicates which queue
handler is registered to which protocol family.  This is useful since
there are now multiple queue handlers in the treee (ip[6]_queue,
nfnetlink_queue).

Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>