Path MTU discovery was slightly broken. I took two ICMP packets

Path MTU discovery was slightly broken.  I took two ICMP packets
to create and change the dynamic route.  This behavior was introduced
in net/route.c rev 1.269 when the gateway route allocation was moved
from rt_setgateway() to _rtalloc().  So rtrequest(RTM_ADD) could
return a route without a valid gateway route.  To fix this, call
rt_setgwroute() from _rtalloc() and rt_setgateway().
OK mpi@