i guess i need a flag bit onto a routing table, for PMTUD blackhole
	detection.  the problem i have is expressed in the following comment
	(sys/netinet/ip_icmp.c):
	/*
	 * XXX:   RTV_MTU is overloaded, since the admin can set it
	 *        to turn off PMTU for a route, and the kernel can
	 *        set it to indicate a serious problem with PMTU
	 *        on a route.  We should be using a separate flag
	 *        for the kernel to indicate this.
	 */

	not sure if PMTUD blackhole detection is a generic sys/net problem,
	or a problem just for sys/netinet.  anyway we have RTV_MTU behavior
	already in sys/net so I guess it is okay to add another bit for
	PMTUD blackhole detection.

	now, does it seem okay to add RTV_xx value that does not correspond to
	members of rt_metrics?

	btw - who is using rmx_pksent in rt_metrics?  what is it for?

itojun