session: Do not expect meaningful errno on non-UNIX

Looking at the g_poll() implementations for various systems, it
appears that on Windows the return value is 0 if the wait was
interrupted, and errno is never set. Also, the MacOS X wrapper
around select() does not clear revents on timeout.

To deal with these issues, check for EINTR only on Unices, and
assume revents to be invalid unless g_poll() returned a positive
value.