Experimental Windows XP support

This is the changes we have made for Windows XP support.

This code is working for us in production, but our use case doesn't heavily stress the nanomsg API.

To resolve the symbols problems, we look for the Vista+ only functions at runtime and use them if available. Otherwise the code runs in XP mode.

We also provide a test that highlights some of the problems we are aware of with the XP path (ipc_xp_test.c).

This should be of interest to issues #330 and #334