We now remember the number of kernel routes from one call of check_xroutes
to the next to avoid calling down into the kernel multiple times.

This very slightly reduces the amount of jitter in some cases.

usleep doesn't handle large arguments portably.

Monotonic time is not stable across reboots.

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

This protects against clock stepping if CLOCK_MONOTONIC is implemented.