To allow a network stack to support both NSAPI and its own options, try to make
sure the NSAPI levels don't collide with level numbers likely to be used by
network stacks.
Distinguish between socket and stack options, and tighten up documentation. Add
IP MRU stack options as an example (implementation not immediately planned for
any stack, but could be useful).
nsapi_error_t - enum of errors or 0 for NSAPI_ERROR_OK
nsapi_size_t - unsigned size of data that could be sent
nsapi_size_or_error_t - either a non-negative size or negative error