net: Convert nf_conntrack_proto to use register_net_sysctl

There isn't much advantage here except that strings paths are a bit
easier to read, and converting everything to them allows me to kill off
ctl_path.

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