Implementation of PIN-based pairing

[bradkeifer]

Initial work to support PIN-based pairing to devices such as Apple TV's.
This is not yet functional.
The next step is to work out where in the code (that we will maintain) we can detect that a PIN is required and then prevent, or skip, playback until the PIN-pairing is completed.
The PIN is to be sent to the command named pipe with the command "PIN=XXXX", where XXXX is the 4 digit pin.

[bradkeifer]

PIN-based pairing and subsequent authorisation key based pairing now fully functional.
After PIN-based pairing is successful, the authorisation key details are published to stderr in the below format

db: db_speaker_save:Device <device_name> has authorization key '<auth_key>'

where:

• <device_name> is the name of the AirPlay2 device; and
• <auth_key> is the authorisation key that needs to be used for future connections in order to prevent the need to supply a new PIN

Support for passing the authorisation key as a command line argument has been added as --auth