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