For link, address, route, add a `WithOptions` variant to the
`*Subscribe()` function to specify a namespace and an error
callback. Those options can be extended in the future without adding
more functions. For example, it could be possible to subscribe only
for a given family by adding a `Family` member to the appropriate
struct.
As a minor change, the private function is always suffixed by `At`,
since it was the case for route and raw netlink functions (but not for
address and link).
The go get command and make both fail when executed on
non-linux platforms. Modified it so that there are no
compilation errors when developing in such an
environment.