LwIP support in POSIX networking code should forward to LwIP functions itself

[eboasson]

Instead of relying on macros that map the well-known BSD socket interface functions to the LwIP names, the code for supporting LwIP should call the native functions directly. That way there is no undue dependency on LwIP configuration in the implementation.

This should be done in the POSIX code, as it really is about a handful of functions and not worth having a separate implementation for LwIP.