RF Space build failure on MSVC

[guruofquality]

Currently RF Space support is disabled in the PothosSDR windows installer. The gr-osmosdr rfspace blocks have some network API incompatibilities that need to be resolved:

• ifdef for winsock headers vs unix headers
• replace close(sock) with closesocket(sock)
  • provide #define closesocket(s) close(s) macro for unix
• Call WSAStartup()

The changes can also be up-streamed to gr-osmosdr official repository. I have no way to test these changes other than getting the module to compile; so if you are interested in RF space support in the Pothos SDR environment please let us know.

Cross platform network includes for reference: https://github.com/pothosware/SoapyRemote/blob/master/common/SoapySocketDefs.in.hpp