net: add basic C code generators for Netlink

Code generators to turn Netlink specs into C code.
I'm definitely not proud of it.

The main generator is in Python, there's a bash script
to regen all code-gen'ed files in tree after making
spec changes.

Acked-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>