netlink: dump interface capabilities with other interface data.

This change exports interface capabilities using the standard
Netlink attribute type, bitset, and switches ifconfig(8) to use
it when displaying interface data.
Bitset comes in two representations. The first one is "compact",
where the bits are exported via two arrays - "mask" listing the
"valid" bits and "values, providing the values for those bits.
The second one is more verbose, listing each bit as a separate item,
with its name, id and value. The latter option is handy when submitting
update requests.

The support for setting capabilities will be added in the upcoming diffs.

Differential Revision: https://reviews.freebsd.org/D40331