Bidirectional PAD communication protocol

The communication between ODR-PadEnc and ODR-AudioEnc was done using a nonblocking FIFO until ODR-PadEnc v3, and was subsequently replaced by a unix domain datagram socket.

The implementations are in https://github.com/Opendigitalradio/ODR-PadEnc/blob/master/src/pad_interface.cpp and https://github.com/Opendigitalradio/ODR-AudioEnc/blob/master/src/PadInterface.cpp