Changes for OS X (tested with OS X 10.9/Mavericks only)

 - use LOCAL_PEERCRED on OS X for checking credentials;
 - don't do setsockopt(SOL_SOCKET, SO_RCVBUF|SO_SNDBUF, 0) on OS X;– things are working properly still anyway, no apparent buffering is present
tox.ini: remove python-signalfd dep