This prevents a stale route from remaining installed when we get a new xroute.

This protects against clock stepping if CLOCK_MONOTONIC is implemented.

Recent kernels don't include RTA_PRIORITY when it's 0.

Recent kernels don't include RTA_DST in the default route.  This fixes
incorrect parsing of such entries.  Reported by Robert Lukan.

It was difficult to write, there's no reason it should easy to read.