iproute: fix unit conversion of rtt/rttvar/rto_min

iproute: fix unit conversion of rtt/rttvar/rto_min

Since July 2008 (2.6.27, c1e20f7c8b9), the kernel stores the values for
RTAX_{RTT{,VAR},RTO_MIN} in milliseconds. When using a kernel > 2.6.27 with
the current iproute2, conversion of these values is broken in either way.

This patch
 * updates the code to pass and retrieve milliseconds;
 * since values < 1msec would be rounded up, also drops the usec/nsec variants;
 * since there is no way to query kernel HZ, also drops the jiffies variant.

Arguments such as
	rtt		3.23sec
	rto_min		0xff
	rto_min		0.200s
	rttvar		25ms
now all work as expected when reading back previously set values.