net: context: clean up net_context_get()

* reduced cyclomatic complexity
* group validation by family to make the validation easier to understand
and extend
* change preprocessor markup where possible to allow for complete code
elimination when features (esp. IP) are disabled
* renamed net_context_get/set_ip_proto() to net_context_get_proto()

While the latter is formally part of the public API and might therefore
have to be deprecated rather than renamed, it is considered internal API
by the net developers, see
https://github.com/zephyrproject-rtos/zephyr/pull/48751#discussion_r942402612



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