Fix source IP address for ICMP unreachable packets generated by tinc.

Try to send ICMP unreachable replies from an address assigned to the
local machine, instead of the destination address of the original
packet.

The address is found by looking up the route towards the sender of
the packet that generated the error; in usual configurations, this
is the tinc interface.

This also fixes the traceroute display in mtr when using the
DecrementTTL option.

Signed-off-by: Vittorio Gambaletta <openwrt@vittgam.net>