libsysdecode: cache Generic Netlink family IDs

Record Generic Netlink family IDs learned from
CTRL_CMD_GETFAMILY responses and use them to decode subsequent
Generic Netlink messages using symbolic family names instead of
numeric IDs.

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