Convert the 2nd rttimer callback from struct rttimer to u_int rtableid.

Convert the 2nd rttimer callback from struct rttimer to u_int rtableid.
The callback only needs to know the rtableid all the other info from
struct rtableid is not needed.
Also change the default rttimer callback to only delete routes that are
RTF_HOST and RTF_DYNAMIC. This way 2 of the ICMP handlers can use NULL
as the callback.
OK bluhm@