netlink: improve interface handling

netlink: improve interface handling

* Separate interface creation from interface modification code
* Support setting some interface attributes (ifdescr, mtu, up/down, promisc)
* Improve interaction with the cloners requiring to parse/write custom
 interface attributes
* Add bitmask-based way of checking if the attribute is present in the
message
* Don't use multipart RTM_GETLINK replies when searching for the
specific interface names
* Use ENODEV instead of ENOENT in case of failed RTM_GETLINK search
* Add python netlink test helpers
* Add some netlink interface tests

Differential Revision: https://reviews.freebsd.org/D37668

(cherry picked from commit 80f03e63d67ede8fedbed4bd6bf6b12ec2ab2cfb)