osx: disable app-nap during serial comm (#687)

Apple introduced app-nap with OS X 10.10. This feature
saves power by throttling apps running in background.
However, it also less accurate timers in systemcalls. In
our case a 30ms select timeout would take up to 10s.

This patch uses an API also added with 10.10 to disable
app-nap as long as the serial port is polled.