ECMP enhancement

One of the advantages of enabling ECMP is to allow for connection load balancing.
Currently the address alias handling method is colliding with the ECMP code.
For example, when two interfaces are configured on the same prefix, only
one prefix route is installed. So connection load balancing is not possible.

The other advantage of ECMP is for failover. The issue with the current code, is 
that the interface link-state is not reflected in the route entry. For example, 
if there are two interfaces on the same prefix, the cable on one interface is 
unplugged, new and existing connections should switch over to the other interface. 
This is not done today and packets go into a black hole.

I discussed about these issues on the list about a month ago.

Also, there is a small bug in the kernel where deleting ECMP routes in the 
userland will always return an error even though the command is successfully
executed.

I have a patch that addresses the above issues. The patch is available at:

   http://people.freebsd.org/~qingli/ecmp-linkstate-patch.diff

This is not the final version. Your comments are welcome.

-- Qing