[NET]: Replace CONFIG_NET_DEBUG with sysctl.

Covert network warning messages from a compile time to runtime choice.
Removes kernel config option and replaces it with new /proc/sys/net/core/warnings.

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/filesystems/proc.txt

@@ -1421,6 +1421,15 @@ fewer messages that will be written. Message_burst controls when messages will
 be dropped.  The  default  settings  limit  warning messages to one every five
 seconds.
 
+warnings
+--------
+
+This controls console messages from the networking stack that can occur because
+of problems on the network like duplicate address or bad checksums. Normally,
+this should be enabled, but if the problem persists the messages can be
+disabled.
+
+
 netdev_max_backlog
 ------------------