Read IP forwarding variables only once.

Read IP forwarding variables only once.

Do not assume that ip_forwarding and ip_directedbcast cannot change
while processing one packet.  Read it once and pass down its value
with a flag.  This is necessary for unlocking the sysctl path.
There are a few places where a consistent value does not really
matter, they are unchanged.  Use a proper ip_ prefix for the global
variable.

OK claudio@