netpfil: add PF netlink command decoding support

Convert PFNL_CMD values in pf_nl.h from an enum to #define constants,
add a pfnl_cmd table definition for mktable, and implement the
corresponding command decoding helpers in libsysdecode.

This allows mktable to generate PF netlink command lookup tables and
enables symbolic decoding of PF netlink commands.

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