net: introduce NET_IP config to efficiently mark conditional code

The code contained several repeated composite IPv4/v6 and UDP/TCP
preprocessor statements that can be simplified by introducing a hidden
NET_IP preprocessor constant that captures what probably is actually
"meant" by this code.

While we were on it we also used the new constant to further isolate
IP-specific code from non-IP specific generics.

Signed-off-by: Florian Grandel <jerico.dev@gmail.com>