libsysdecode: decode combined Netlink message flags

Change sysdecode_nlm_flag() to decode Netlink message flags as a
bitmask instead of looking up a single flag value. This correctly
prints combinations of NLM_F_* flags while preserving any unknown
bits in hexadecimal.

Reported by: androvonx95 <androvonx95@tutamail.com>
Reviewed by: kp
Signed-off-by: Ishan Agrawal <iagrawal9990@gmail.com>
Sponsored-by: Google LLC (GSoC 2026)