Fixes #458.

Add many build options to the NetBSD target
Set any warnings to be errors as NetBSD should build without
any warnings. DragonFly as well, but we don't have a target for that.

Because some OS put the interface MTU into the route MTU if the
route does not define its own.

Netmask family and length are ignored by traditional
userland tools such as route and netstat and are assumed
to match the destination sockaddr.
This is fortunate because BSD kernels use a radix tree
to store routes which adjusts the netmask at the point
of insertion where this information is lost.
We can just sub in the values from the destination address.

This is currently true for all BSD kernels.

We need to make two sysctls to pull the route table from the kernel.
The first one works out the size of the buffer required and the
second one populates it.
It's possible for more routes to be added between these two calls
causing the second call to fail with ENOMEM.
If this happens, just re-query the size needed and try again.

Fixes #466.

Each lifetime has a an aquistion time so we can compare to
now to know how much of the lifetime is left.
Apply the difference when adding/changing the route just as we
do for addresses.

This allow us to work out better when we need to update the
route lifetime from a newer aquisition.

Fixes #465.

Fixes #472.