Modularization of the doorbell program to support remote buttons and bells

[bradxn]

The DingDong() function in gpio.cpp should be a separate program (shell script actually).

However, gpio.cp should not call DingDong()! Instead, it should broadcast an SSDP NOTIFY message indicating that the doorbell button has been pressed.

A new SSDP listening daemon should be written that listens for NOTIFY messages. When one is received, the NT and NTS fields should be looked up in a config file that may show a program to run. The config file should indicate that the DingDong program should be run for the appropriate doorbell button notification.

This will allow a remote system to also monitor NOTIFY messages and ring its own bell as well as remote programs to send button press NOTIFY messages (like the tablet doorbell button program).