libsysdecode: decode PF Generic Netlink commands

Decode the Generic Netlink command header for messages
belonging to the PF Generic Netlink family. Display the
command name using the PF Generic Netlink command decoder.

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