truss: add AF_NETLINK syscall argument decoding support

Track AF_NETLINK socket file descriptors in procinfo using fd_domain
entries and use them to detect Netlink traffic across later syscalls.

Add Netlink payload decoding support for sendto(), recvfrom(), and
sendmsg() paths by integrating sysdecode_netlink() into BinString and
Msghdr argument handling, with fallback to existing binary/iovec
printing when decoding fails.

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/2295