Improve abstraction of networking

[eboasson]

The current abstraction still leaks some details into the core DDSI code, among others:

• definitions in platform header files that sometimes clash (e.g., "close" in the entity functions with a macro mapping to lwip_close in LwIP);
• address formats, in particular there are a few spots where IP addressing details are used, but that should be format-independent;
• relationship between sockets & connections — one thing in particular that is impossible today because of the IO multiplexing is having a connection that consists of multiple sockets;
• some code that really deals with sockets but that nonetheless still lives in the core code (q_nwif.c)

There may be more, but these all are good reasons why a round of cleaning up is needed