netlink: decode netlink message flags symbolically

Generate an nlm_flag table definition for mktable from
netlink/netlink.h, add a sysdecode_nlm_flag() helper to
libsysdecode, and use it when decoding netlink message headers.

This enables mktable to generate netlink message flag lookup tables and
replaces raw hexadecimal output for recognized NLM_F_* flag values
with their symbolic names.

Reviewed by: kp
Signed-off-by: Ishan Agrawal <iagrawal9990@gmail.com>
Sponsored by: Google LLC (GSoC 2026)
Pull Request: https://github.com/freebsd/freebsd-src/pull/2294