[IPV4]: Sysctl configurable icmp error source address.

This patch alows you to change the source address of icmp error
messages. It applies cleanly to 2.6.11.11 and retains the default
behaviour.

In the old (default) behaviour icmp error messages are sent with the ip
of the exiting interface.

The new behaviour (when the sysctl variable is toggled on), it will send
the message with the ip of the interface that received the packet that
caused the icmp error. This is the behaviour network administrators will
expect from a router. It makes debugging complicated network layouts
much easier. Also, all 'vendor routers' I know of have the later
behaviour.

Signed-off-by: David S. Miller <davem@davemloft.net>