net: icmp: add option to suppress destination unreachable errors.

By default ICMP desination unreachable error packets are generated when
input packets target ports that are not in a listening state. This not
only reveals the presence of the host on the network which may be
considered a security vulnerability depending on the application, it
also ends up triggering ARP lookups to respond to the sending host. With
a small ARP table and a network where there may be broadcast (or
multicast) service discovery traffic such as mDNS or uPnP, ARP table
thrashing can occur impacting network stack performance.

Signed-off-by: Berend Ozceri <berend@recogni.com>