Avoid SerialPort IOLoop when there's no event handlers

[MrJul]

This PR avoids starting the SerialPort I/O loop uncondtionally on Unix.

dotnet/corefx#40383 added support for the PinChanged event on the Unix version of SerialStream.
However, since SerialPort was always subscribing to SerialStream.PinChanged, this had the side effect of always starting the IOLoop, even when no events were actually subscribed on the SerialPort.

This PR changes this behavior. SerialStream.PinChanged is only subscribed to if SerialPort.PinChanged has handlers.
This mimics the logic for the DataReceived event.

When no events are subscribed, opening a SerialPort without reading or writing now doesn't use any CPU.

First part of fixing #2379 (CPU usage still remains high when reading/writing).

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