[IPV4]: Cleanup the sysctl_net_ipv4.c file

This includes several cleanups:

 * tune Makefile to compile out this file when SYSCTL=n. Now
   it looks like net/core/sysctl_net_core.c one;
 * move the ipv4_config to af_inet.c to exist all the time;
 * remove additional sysctl_ip_nonlocal_bind declaration
   (it is already declared in net/ip.h);
 * remove no nonger needed ifdefs from this file.

This is a preparation for using ctl paths for net/ipv4/
sysctl table.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>