tools: ynl: create local attribute helpers

Don't use mnl attr helpers, we're trying to remove the libmnl
dependency. Create both signed and unsigned helpers, libmnl
had unsigned helpers, so code generator no longer needs
the mnl_type() hack.

The new helpers are written from first principles, but are
hopefully not too buggy.

Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Link: https://lore.kernel.org/r/20240227223032.1835527-3-kuba@kernel.org


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