io/command: implement portable spawn Hi, This enables support of command spawning in the io/ subsystem on Windows. v4: - add "util: make do_send_recv work with partial send/recv" patch - drop usage of SOCAT environment variable for tests - misc style fixes v3: - actually make it work :) (it seems I didn't test correctly earlier attempts) - add a patch for readv_writev() and partial IOs - add a patch to avoid some potential race in echo test - drop usage of PIPE_NOWAIT which was broken