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.