tools: ynl: create local nlmsg access helpers

Create helpers for accessing payloads of struct nlmsg.
Use them instead of the libmnl ones.

Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Link: https://lore.kernel.org/r/20240227223032.1835527-5-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>