libsysdecode: decode Generic Netlink controller messages

Decode Generic Netlink controller (GENL_ID_CTRL) messages in
Netlink payloads. Display the Generic Netlink header along with
the CTRL_CMD_GETFAMILY attributes, including the family ID and
family name.

Signed-off-by: Ishan Agrawal <iagrawal9990@gmail.com>
Reviewed by: kp
Sponsored-by: Google LLC (GSoC 2026)